erofs: clean up z_erofs_pcluster_readmore()

`end` parameter is no needed since it's pointless for !backmost, we can
handle it with backmost internally.  And we only expand the trailing
edge, so the newstart can be replaced with ->headoffset.

Also, remove linux/prefetch.h inclusion since that is not used anymore
after commit 38629291 ("erofs: introduce readmore decompression
strategy").
Signed-off-by: Yue Hu <huyue2@coolpad.com>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Link: https://lore.kernel.org/r/20230525072605.17857-1-zbestahu@gmail.com
[ Gao Xiang: update commit description. ]
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/zdata.c

@@ -5,7 +5,6 @@
  * Copyright (C) 2022 Alibaba Cloud
  */
 #include "compress.h"
-#include <linux/prefetch.h>
 #include <linux/psi.h>
 #include <linux/cpuhotplug.h>
 #include <trace/events/erofs.h>
@@ -1825,28 +1824,28 @@ static void z_erofs_runqueue(struct z_erofs_decompress_frontend *f,
  */
 static void z_erofs_pcluster_readmore(struct z_erofs_decompress_frontend *f,
 				      struct readahead_control *rac,
-				      erofs_off_t end,
-				      struct page **pagepool,
-				      bool backmost)
+				      struct page **pagepool, bool backmost)
 {
 	struct inode *inode = f->inode;
 	struct erofs_map_blocks *map = &f->map;
-	erofs_off_t cur;
+	erofs_off_t cur, end, headoffset = f->headoffset;
 	int err;
 
 	if (backmost) {
+		if (rac)
+			end = headoffset + readahead_length(rac) - 1;
+		else
+			end = headoffset + PAGE_SIZE - 1;
 		map->m_la = end;
 		err = z_erofs_map_blocks_iter(inode, map,
 					      EROFS_GET_BLOCKS_READMORE);
 		if (err)
 			return;
 
-		/* expend ra for the trailing edge if readahead */
+		/* expand ra for the trailing edge if readahead */
 		if (rac) {
-			loff_t newstart = readahead_pos(rac);
-
 			cur = round_up(map->m_la + map->m_llen, PAGE_SIZE);
-			readahead_expand(rac, newstart, cur - newstart);
+			readahead_expand(rac, headoffset, cur - headoffset);
 			return;
 		}
 		end = round_up(end, PAGE_SIZE);
@@ -1894,10 +1893,9 @@ static int z_erofs_read_folio(struct file *file, struct folio *folio)
 	trace_erofs_readpage(page, false);
 	f.headoffset = (erofs_off_t)page->index << PAGE_SHIFT;
 
-	z_erofs_pcluster_readmore(&f, NULL, f.headoffset + PAGE_SIZE - 1,
-				  &pagepool, true);
+	z_erofs_pcluster_readmore(&f, NULL, &pagepool, true);
 	err = z_erofs_do_read_page(&f, page, &pagepool);
-	z_erofs_pcluster_readmore(&f, NULL, 0, &pagepool, false);
+	z_erofs_pcluster_readmore(&f, NULL, &pagepool, false);
 
 	(void)z_erofs_collector_end(&f);
 
@@ -1923,8 +1921,7 @@ static void z_erofs_readahead(struct readahead_control *rac)
 
 	f.headoffset = readahead_pos(rac);
 
-	z_erofs_pcluster_readmore(&f, rac, f.headoffset +
-				  readahead_length(rac) - 1, &pagepool, true);
+	z_erofs_pcluster_readmore(&f, rac, &pagepool, true);
 	nr_pages = readahead_count(rac);
 	trace_erofs_readpages(inode, readahead_index(rac), nr_pages, false);
 
@@ -1947,7 +1944,7 @@ static void z_erofs_readahead(struct readahead_control *rac)
 				  page->index, EROFS_I(inode)->nid);
 		put_page(page);
 	}
-	z_erofs_pcluster_readmore(&f, rac, 0, &pagepool, false);
+	z_erofs_pcluster_readmore(&f, rac, &pagepool, false);
 	(void)z_erofs_collector_end(&f);
 
 	z_erofs_runqueue(&f, &pagepool,