fs/proc/task_mmu.c: suppress compilation warnings with W=1

Suppress a bunch of warnings of the form:

  fs/proc/task_mmu.c: In function 'show_smap_vma_flags':
  fs/proc/task_mmu.c:635:22: warning: initialized field overwritten [-Wt override-init]
     [ilog2(VM_READ)] = "rd",
                        ^~~~
  fs/proc/task_mmu.c:635:22: note: (near initialization for 'mnemonics[0]')

They happen because of the way we intentionally build the table, so
silence the warning when building with 'make W=1'.

Link: http://lkml.kernel.org/r/8727.1470022083@turing-police.cc.vt.edu
Signed-off-by: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/Makefile

@@ -4,6 +4,7 @@
 
 obj-y   += proc.o
 
+CFLAGS_task_mmu.o	+= -Wno-override-init
 proc-y			:= nommu.o task_nommu.o
 proc-$(CONFIG_MMU)	:= task_mmu.o