word-at-a-time: provide generic big-endian zero_bytemask implementation

word-at-a-time: provide generic big-endian zero_bytemask implementation

Whilst architectures may be able to do better than this (which they can,
by simply defining their own macro), this is a generic stab at a
zero_bytemask implementation for the asm-generic, big-endian
word-at-a-time implementation.

On arm64, a clz instruction is used to implement the fls efficiently.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
(cherry picked from commit 11ec50caedb56e3a87715edeff6a1852e6ae5416)
Signed-off-by: Mark Brown <broonie@linaro.org>