9PFS: clean up explicit check for mandatory locks

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

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Latchesar Ionkov <lucho@ionkov.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>