GFS2: Fix crash during ACL deletion in acl max entry check in gfs2_set_acl()

Fixes: e01580bf ("gfs2: use generic posix ACL infrastructure")
Reported-by: Eric Meddaugh <etmsys@rit.edu>
Tested-by: Eric Meddaugh <etmsys@rit.edu>
Signed-off-by: Andrew Elble <aweits@rit.edu>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>