erofs: refine readahead tracepoint

- trace_erofs_readpages => trace_erofs_readahead;

 - Rename a redundant statement `nrpages = readahead_count(rac);`;

 - Move the tracepoint to the beginning of z_erofs_readahead().

Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Hongbo Li <lihongbo22@huawei.com>
Link: https://lore.kernel.org/r/20250514120820.2739288-1-hsiangkao@linux.alibaba.com
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>

fs/erofs/fileio.c

@@ -180,7 +180,7 @@ static void erofs_fileio_readahead(struct readahead_control *rac)
 	struct folio *folio;
 	int err;
 
-	trace_erofs_readpages(inode, readahead_index(rac),
+	trace_erofs_readahead(inode, readahead_index(rac),
 			      readahead_count(rac), true);
 	while ((folio = readahead_folio(rac))) {
 		err = erofs_fileio_scan_folio(&io, folio);

fs/erofs/zdata.c

@@ -1855,13 +1855,12 @@ static void z_erofs_readahead(struct readahead_control *rac)
 {
 	struct inode *const inode = rac->mapping->host;
 	Z_EROFS_DEFINE_FRONTEND(f, inode, readahead_pos(rac));
-	struct folio *head = NULL, *folio;
 	unsigned int nrpages = readahead_count(rac);
+	struct folio *head = NULL, *folio;
 	int err;
 
+	trace_erofs_readahead(inode, readahead_index(rac), nrpages, false);
 	z_erofs_pcluster_readmore(&f, rac, true);
-	nrpages = readahead_count(rac);
-	trace_erofs_readpages(inode, readahead_index(rac), nrpages, false);
 	while ((folio = readahead_folio(rac))) {
 		folio->private = head;
 		head = folio;

include/trace/events/erofs.h

@@ -113,7 +113,7 @@ TRACE_EVENT(erofs_read_folio,
 		__entry->raw)
 );
 
-TRACE_EVENT(erofs_readpages,
+TRACE_EVENT(erofs_readahead,
 
 	TP_PROTO(struct inode *inode, pgoff_t start, unsigned int nrpage,
 		bool raw),