f2fs: use spinlock rather than mutex for better speed

With the 2 previous changes, all the long time operations are moved out
of the protection region, so here we can use spinlock rather than mutex
(orphan_inode_mutex) for lower overhead.

Signed-off-by: Gu Zheng <guz.fnst@cn.fujitsu.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 8466b5e..ee304fb 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -412,7 +412,7 @@
 
 	/* for orphan inode management */
 	struct list_head orphan_inode_list;	/* orphan inode list */
-	struct mutex orphan_inode_mutex;	/* for orphan inode list */
+	spinlock_t orphan_inode_lock;		/* for orphan inode list */
 	unsigned int n_orphans;			/* # of orphan inodes */
 	unsigned int max_orphans;		/* max orphan inodes */