f2fs: use cow inode data when updating atomic write

Need to use cow inode data content instead of the one in the original
inode, when we try to write the already updated atomic write files.

Signed-off-by: Daeho Jeong <daehojeong@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>