f2fs: split f2fs_allocate_new_segments()

to two independent functions:
- f2fs_allocate_new_segment() for specified type segment allocation
- f2fs_allocate_new_segments() for all data type segments allocation

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>