f2fs: fix to check return value __allocate_new_segment

__allocate_new_segment may return error when get_new_segment
fails, so its caller should check its return value.

Signed-off-by: Zhiguo Niu <zhiguo.niu@unisoc.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>