fanotify: do not dereference inode_mark when it is unset

The fanotify code is supposed to get the group from the mark.  It accidentally
only used the inode_mark.  If the vfsmount_mark was set but not the inode_mark
it would deref the NULL inode_mark.  Get the group from the correct place.

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@sophos.com>
Signed-off-by: Eric Paris <eparis@redhat.com>