erofs: introduce z_erofs_do_decompressed_bvec()

Both out_bvecs and in_bvecs share the common logic for decompressed
buffers. So let's make a helper for this.

Acked-by: Chao Yu <chao@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220715154203.48093-15-hsiangkao@linux.alibaba.com