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>