lib/string.c: introduce memchr_inv()

lib/string.c: introduce memchr_inv()

memchr_inv() is mainly used to check whether the whole buffer is filled
with just a specified byte.

The function name and prototype are stolen from logfs and the
implementation is from SLUB.

Signed-off-by: Akinobu Mita <[email protected]>
Acked-by: Christoph Lameter <[email protected]>
Acked-by: Pekka Enberg <[email protected]>
Cc: Matt Mackall <[email protected]>
Acked-by: Joern Engel <[email protected]>
Cc: Marcin Slusarz <[email protected]>
Cc: Eric Dumazet <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>