f2fs: add a f2fs_lookup_extent_cache_block helper

All but three callers of f2fs_lookup_extent_cache just want the block
address.  Add a small helper to simplify them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>