GFS2: use memchr_inv

Use memchr_inv to verify that the specified memory range is cleared.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Cc: cluster-devel@redhat.com
Cc: Christine Caulfield <ccaulfie@redhat.com>
Cc: David Teigland <teigland@redhat.com>