f2fs: fix to check current blkaddr in __allocate_data_blocks

In __allocate_data_blocks, we should check current blkaddr which is located at
ofs_in_node of dnode page instead of checking first blkaddr all the time.
Otherwise we can only allocate one blkaddr in each dnode page. Fix it.

Signed-off-by: Chao Yu <chao2.yu@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>