bufferhead: force inlining of buffer head flag operations

With both gcc 4.7.2 and 4.9.2, sometimes gcc mysteriously doesn't inline
very small functions we expect to be inlined.  See

    https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66122

With this .config:
http://busybox.net/~vda/kernel_config_OPTIMIZE_INLINING_and_Os,
set_buffer_foo(), clear_buffer_foo() and similar functions get deinlined
about 60 times. Examples of disassembly:

<set_buffer_mapped> (14 copies, 43 calls):
       55                      push   %rbp
       48 89 e5                mov    %rsp,%rbp
       f0 80 0f 20             lock orb $0x20,(%rdi)
       5d                      pop    %rbp
       c3                      retq
<buffer_mapped> (3 copies, 34 calls):
       48 8b 07                mov    (%rdi),%rax
       55                      push   %rbp
       48 89 e5                mov    %rsp,%rbp
       48 c1 e8 05             shr    $0x5,%rax
       83 e0 01                and    $0x1,%eax
       5d                      pop    %rbp
       c3                      retq
<set_buffer_new> (5 copies, 13 calls):
       55                      push   %rbp
       48 89 e5                mov    %rsp,%rbp
       f0 80 0f 40             lock orb $0x40,(%rdi)
       5d                      pop    %rbp
       c3                      retq

This patch fixes this via s/inline/__always_inline/.
This decreases vmlinux by about 3 kbytes.

    text	    data	     bss	      dec	    hex	filename
88200439	19905208	36421632	144527279	89d4faf	vmlinux2
88197239	19905240	36421632	144524111	89d434f	vmlinux

Signed-off-by: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Graf <tgraf@suug.ch>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/buffer_head.h

@@ -82,15 +82,15 @@ struct buffer_head {
  * and buffer_foo() functions.
  */
 #define BUFFER_FNS(bit, name)						\
-static inline void set_buffer_##name(struct buffer_head *bh)		\
+static __always_inline void set_buffer_##name(struct buffer_head *bh)	\
 {									\
 	set_bit(BH_##bit, &(bh)->b_state);				\
 }									\
-static inline void clear_buffer_##name(struct buffer_head *bh)		\
+static __always_inline void clear_buffer_##name(struct buffer_head *bh)	\
 {									\
 	clear_bit(BH_##bit, &(bh)->b_state);				\
 }									\
-static inline int buffer_##name(const struct buffer_head *bh)		\
+static __always_inline int buffer_##name(const struct buffer_head *bh)	\
 {									\
 	return test_bit(BH_##bit, &(bh)->b_state);			\
 }
@@ -99,11 +99,11 @@ static inline int buffer_##name(const struct buffer_head *bh)		\
  * test_set_buffer_foo() and test_clear_buffer_foo()
  */
 #define TAS_BUFFER_FNS(bit, name)					\
-static inline int test_set_buffer_##name(struct buffer_head *bh)	\
+static __always_inline int test_set_buffer_##name(struct buffer_head *bh) \
 {									\
 	return test_and_set_bit(BH_##bit, &(bh)->b_state);		\
 }									\
-static inline int test_clear_buffer_##name(struct buffer_head *bh)	\
+static __always_inline int test_clear_buffer_##name(struct buffer_head *bh) \
 {									\
 	return test_and_clear_bit(BH_##bit, &(bh)->b_state);		\
 }									\