f2fs: update several comments

Update several comments:
1. use f2fs_{un}lock_op install of mutex_{un}lock_op.
2. update comment of get_data_block().
3. update description of node offset.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>
diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index 42f28d4..ba91186 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -165,7 +165,7 @@
 	LOOKUP_NODE,			/* look up a node without readahead */
 	LOOKUP_NODE_RA,			/*
 					 * look up a node with readahead called
-					 * by get_datablock_ro.
+					 * by get_data_block.
 					 */
 };