make asm-generic/cacheflush.h more standalone

Some s390 builds get these warnings:

  include/asm-generic/cacheflush.h:16:42: warning: 'struct mm_struct' declared inside parameter list will not be visible outside of this definition or declaration
  include/asm-generic/cacheflush.h:22:46: warning: 'struct mm_struct' declared inside parameter list will not be visible outside of this definition or declaration
  include/asm-generic/cacheflush.h:28:45: warning: 'struct vm_area_struct' declared inside parameter list will not be visible outside of this definition or declaration
  include/asm-generic/cacheflush.h:36:44: warning: 'struct vm_area_struct' declared inside parameter list will not be visible outside of this definition or declaration
  include/asm-generic/cacheflush.h:44:45: warning: 'struct page' declared inside parameter list will not be visible outside of this definition or declaration
  include/asm-generic/cacheflush.h:52:50: warning: 'struct address_space' declared inside parameter list will not be visible outside of this definition or declaration
  include/asm-generic/cacheflush.h:58:52: warning: 'struct address_space' declared inside parameter list will not be visible outside of this definition or declaration
  include/asm-generic/cacheflush.h:75:17: warning: 'struct page' declared inside parameter list will not be visible outside of this definition or declaration
  include/asm-generic/cacheflush.h:74:45: warning: 'struct vm_area_struct' declared inside parameter list will not be visible outside of this definition or declaration
  include/asm-generic/cacheflush.h:82:16: warning: 'struct page' declared inside parameter list will not be visible outside of this definition or declaration
  include/asm-generic/cacheflush.h:81:50: warning: 'struct vm_area_struct' declared inside parameter list will not be visible outside of this definition or declaration

Forward declare the named structs to get rid of these.

Link: http://lkml.kernel.org/r/20200623135714.4dae4b8a@canb.auug.org.au
Fixes: e0cf615d72 ("asm-generic: don't include <linux/mm.h> in cacheflush.h")
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/cacheflush.h

@@ -2,6 +2,11 @@
 #ifndef _ASM_GENERIC_CACHEFLUSH_H
 #define _ASM_GENERIC_CACHEFLUSH_H
 
+struct mm_struct;
+struct vm_area_struct;
+struct page;
+struct address_space;
+
 /*
  * The cache doesn't need to be flushed when TLB entries change when
  * the cache is mapped to physical memory, not virtual memory