LU-12275 sec: use memchr_inv() to check if page is zero.

LU-12275 sec: use memchr_inv() to check if page is zero.

memchr_inv() is the preferred way to check if a memory region is all
zeros.  It is likely fast that memcmp() is it doesn't need to read the
ZERO_PAGE into cache, or into the CPU.  It was introduced in Linux
3.2.

Test-Parameters: trivial
Signed-off-by: Mr NeilBrown <neilb@suse.de>
Change-Id: I0a5c3d30d5db43a3f5ebb270ea66b9db2b200a9a
Reviewed-on: https://review.whamcloud.com/39459
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Sebastien Buisson <sbuisson@ddn.com>