erofs: simplify z_erofs_transform_plain()

Use memcpy_to_page() instead of open-coding them.

In addition, add a missing flush_dcache_page() even though almost all
modern architectures clear `PG_dcache_clean` flag for new file cache
pages so that it doesn't change anything in practice.
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Link: https://lore.kernel.org/r/20230627161240.331-2-hsiangkao@linux.alibaba.comSigned-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/decompressor.c

@@ -328,7 +328,7 @@ static int z_erofs_transform_plain(struct z_erofs_decompress_req *rq,
 	const unsigned int lefthalf = rq->outputsize - righthalf;
 	const unsigned int interlaced_offset =
 		rq->alg == Z_EROFS_COMPRESSION_SHIFTED ? 0 : rq->pageofs_out;
-	unsigned char *src, *dst;
+	u8 *src;
 
 	if (outpages > 2 && rq->alg == Z_EROFS_COMPRESSION_SHIFTED) {
 		DBG_BUGON(1);
@@ -341,22 +341,19 @@ static int z_erofs_transform_plain(struct z_erofs_decompress_req *rq,
 	}
 
 	src = kmap_local_page(rq->in[inpages - 1]) + rq->pageofs_in;
-	if (rq->out[0]) {
-		dst = kmap_local_page(rq->out[0]);
-		memcpy(dst + rq->pageofs_out, src + interlaced_offset,
-		       righthalf);
-		kunmap_local(dst);
-	}
+	if (rq->out[0])
+		memcpy_to_page(rq->out[0], rq->pageofs_out,
+			       src + interlaced_offset, righthalf);
 
 	if (outpages > inpages) {
 		DBG_BUGON(!rq->out[outpages - 1]);
 		if (rq->out[outpages - 1] != rq->in[inpages - 1]) {
-			dst = kmap_local_page(rq->out[outpages - 1]);
-			memcpy(dst, interlaced_offset ? src :
-					(src + righthalf), lefthalf);
-			kunmap_local(dst);
+			memcpy_to_page(rq->out[outpages - 1], 0, src +
+					(interlaced_offset ? 0 : righthalf),
+				       lefthalf);
 		} else if (!interlaced_offset) {
 			memmove(src, src + righthalf, lefthalf);
+			flush_dcache_page(rq->in[inpages - 1]);
 		}
 	}
 	kunmap_local(src);