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

The goal of fs_changed() is to check whether the tree changed during a
schedule(). This is a BKL legacy.

A recent patch added an explicit unconditional release/reacquire of the
write lock around the cond_resched() called inside fs_changed.

But it's wasteful to unconditionally do that, we are creating superfluous
lock contention in !TIF_NEED_RESCHED case.

This patch manage that by calling reiserfs_cond_resched() from fs_changed()
which only releases the lock if we are going to reschedule.

[ Impact: inject less lock contention and tree job retries ]

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

include/linux/reiserfs_fs.h

@@ -1348,9 +1348,7 @@ static inline loff_t max_reiserfs_offset(struct inode *inode)
 #define __fs_changed(gen,s) (gen != get_generation (s))
 #define fs_changed(gen,s)		\
 ({					\
-	reiserfs_write_unlock(s);	\
-	cond_resched();			\
-	reiserfs_write_lock(s);		\
+	reiserfs_cond_resched(s);	\
 	__fs_changed(gen, s);		\
 })