f2fs: avoid using native allocate_segment_by_default()

As we did for other cases, in fix_curseg_write_pointer(), let's
use wrapped f2fs_allocate_new_section() instead of native
allocate_segment_by_default(), by this way, it fixes to cover
segment allocation with curseg_lock and sentry_lock.

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