dcache: allow word-at-a-time name hashing with big-endian CPUs

When explicitly hashing the end of a string with the word-at-a-time
interface, we have to be careful which end of the word we pick up.

On big-endian CPUs, the upper-bits will contain the data we're after, so
ensure we generate our masks accordingly (and avoid hashing whatever
random junk may have been sitting after the string).

This patch adds a new dcache helper, bytemask_from_count, which creates
a mask appropriate for the CPU endianness.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/dcache.h

@@ -29,8 +29,10 @@ struct vfsmount;
 /* The hash is always the low bits of hash_len */
 #ifdef __LITTLE_ENDIAN
  #define HASH_LEN_DECLARE u32 hash; u32 len;
+ #define bytemask_from_count(cnt)	(~(~0ul << (cnt)*8))
 #else
  #define HASH_LEN_DECLARE u32 len; u32 hash;
+ #define bytemask_from_count(cnt)	(~(~0ul >> (cnt)*8))
 #endif
 
 /*