GFS2: clean up explicit check for mandatory locks

The __mandatory_lock(inode) function makes the same check, but makes the code
more readable.

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Cc: Steven Whitehouse <swhiteho@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>