z_erofs_pcluster_begin(): don't bother with rounding position down

... and be more idiomatic when calculating ->pageofs_in.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/r/20240425200017.GF1031757@ZenIV
[ Gao Xiang: don't use `offset_in_page(mptr)` due to EROFS_NO_KMAP. ]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

Author: Al Viro

fs/erofs/zdata.c

@@ -868,7 +868,7 @@ static int z_erofs_pcluster_begin(struct z_erofs_decompress_frontend *fe)
 	} else {
 		void *mptr;
 
-		mptr = erofs_read_metabuf(&map->buf, sb, erofs_pos(sb, blknr), EROFS_NO_KMAP);
+		mptr = erofs_read_metabuf(&map->buf, sb, map->m_pa, EROFS_NO_KMAP);
 		if (IS_ERR(mptr)) {
 			ret = PTR_ERR(mptr);
 			erofs_err(sb, "failed to get inline data %d", ret);