nilfs2: do not use yield()

Use cond_resched() instead of yield() in the loop of
nilfs_transaction_lock() since the usage corresponds to the "be nice for
others" case that the comment of yield() says.

This removes the following checkpatch.pl warning:

 "WARNING: Using yield() is generally wrong. See yield() kernel-doc
  (sched/core.c)"

Link: http://lkml.kernel.org/r/1464875891-5443-8-git-send-email-konishi.ryusuke@lab.ntt.co.jp
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/nilfs2/segment.c

@@ -373,7 +373,7 @@ static void nilfs_transaction_lock(struct super_block *sb,
 		nilfs_segctor_do_immediate_flush(sci);
 
 		up_write(&nilfs->ns_segctor_sem);
-		yield();
+		cond_resched();
 	}
 	if (gcflag)
 		ti->ti_flags |= NILFS_TI_GC;