[JFFS2] Fix hanging close for /dev/mtd character device.

When pdflush is erasing lots of sectors, drivers calling
mtd->sync will hang until all blocks are erased. Be nicer.
Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>

fs/jffs2/erase.c

@@ -142,7 +142,7 @@ void jffs2_erase_pending_blocks(struct jffs2_sb_info *c, int count)
 		}
 
 		/* Be nice */
-		cond_resched();
+		yield();
 		spin_lock(&c->erase_completion_lock);
 	}