f2fs: factor a f2fs_map_blocks_cached helper

[ Upstream commit 0094e98b ]

Add a helper to deal with everything needed to return a f2fs_map_blocks
structure based on a lookup in the extent cache.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Stable-dep-of: 9d5c4f5c ("f2fs: fix wrong block mapping for multi-devices")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>