kill-the-BKL/reiserfs: release write lock on fs_changed()

fs_changed() is a macro used by reiserfs to check whether its tree has been
rebalanced. It has been designed to check parallel changes on the tree after
calling a sleeping function, which released the Bkl.

fs_changed() also calls cond_resched(), so that if rescheduling is needed,
we are in the best place to do that, since we check if the tree has changed
just after (because of the bkl release on schedule()).

Even if we are not anymore using the Bkl, we still want to release the lock
while we reschedule, so that other waiters for the lock can acquire it safely,
because of the following __fs_changed() check.

[ Impact: release the reiserfs write lock when it is not needed ]

Cc: Jeff Mahoney <jeffm@suse.com>
Cc: Chris Mason <chris.mason@oracle.com>
Cc: Alexander Beregalov <a.beregalov@gmail.com>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>