This patch adds correct error handling codes when get_dnode_of_data() returns
an error.
At first, f2fs_reserve_block() calls f2fs_put_dnode() whenever reserve_new_block
returns an error.
So, the rule of f2fs_reserve_block() is to nullify inode page when there is any
error internally.
Finally, two callers of f2fs_reserve_block() should call f2fs_put_dnode()
appropriately if they got an error since successful f2fs_reserve_block().
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>