f2fs: clean up data_blkaddr() and get_dnode_addr()

Introudce a new help get_dnode_base() to wrap common code from
get_dnode_addr() and data_blkaddr() for cleanup.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>