f2fs: introduce universal lookup/update interface for extent cache

In this patch, we do these jobs:
1. rename {check,update}_extent_cache to {lookup,update}_extent_info;
2. introduce universal lookup/update interface of extent cache:
f2fs_{lookup,update}_extent_cache including above two real functions, then
export them to function callers.

So after above cleanup, we can add new rb-tree based extent cache into exported
interfaces.

v2:
 o remove "f2fs_" for inner function {lookup,update}_extent_info suggested by
   Jaegeuk Kim.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
diff --git a/fs/f2fs/file.c b/fs/f2fs/file.c
index 98dac27..baadaf2 100644
--- a/fs/f2fs/file.c
+++ b/fs/f2fs/file.c
@@ -433,7 +433,7 @@
 			continue;
 
 		dn->data_blkaddr = NULL_ADDR;
-		update_extent_cache(dn);
+		f2fs_update_extent_cache(dn);
 		invalidate_blocks(sbi, blkaddr);
 		nr_free++;
 	}