f2fs: introduce get_dnode_addr() to clean up codes

[ Upstream commit 2020cd48 ]

Just cleanup, no logic changes.

Signed-off-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Stable-dep-of: 54607494 ("f2fs: compress: fix to avoid inconsistence bewteen i_blocks and dnode")
Signed-off-by: Sasha Levin <sashal@kernel.org>