f2fs: enhance lookup xattr

Previously, in getxattr we will load all entries both in inline xattr and
xattr node block, and then do the lookup in all entries, but our lookup
flow shows low efficiency, since if we can lookup and hit in inline xattr
of inode page cache first, we don't need to load and lookup xattr node
block, which can obviously save cpu time and IO latency.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: initialize NULL to avoid warning]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>