f2fs: remove struct segment_allocation default_salloc_ops

There is only  single instance of these ops, so remove the indirection
and call allocate_segment_by_default directly.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>