netfs: drop usage of folio_file_pos

folio_file_pos is only needed for mixed usage of page cache and swap
cache, for pure page cache usage, the caller can just use folio_pos
instead.

It can't be a swap cache page here.  Swap mapping may only call into fs
through swap_rw and that is not supported for netfs.  So just drop it and
use folio_pos instead.

Link: https://lkml.kernel.org/r/20240521175854.96038-7-ryncsn@gmail.comSigned-off-by: Kairui Song <kasong@tencent.com>
Cc: David Howells <dhowells@redhat.com>
Cc: Jeff Layton <jlayton@kernel.org>
Cc: Anna Schumaker <anna@kernel.org>
Cc: Barry Song <v-songbaohua@oppo.com>
Cc: Chao Yu <chao@kernel.org>
Cc: Chris Li <chrisl@kernel.org>
Cc: David Hildenbrand <david@redhat.com>
Cc: "Huang, Ying" <ying.huang@intel.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ilya Dryomov <idryomov@gmail.com>
Cc: Jaegeuk Kim <jaegeuk@kernel.org>
Cc: Marc Dionne <marc.dionne@auristor.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Minchan Kim <minchan@kernel.org>
Cc: NeilBrown <neilb@suse.de>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Cc: Trond Myklebust <trond.myklebust@hammerspace.com>
Cc: Xiubo Li <xiubli@redhat.com>
Cc: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

fs/netfs/buffered_read.c

@@ -271,7 +271,7 @@ int netfs_read_folio(struct file *file, struct folio *folio)
 	_enter("%lx", folio->index);
 
 	rreq = netfs_alloc_request(mapping, file,
-				   folio_file_pos(folio), folio_size(folio),
+				   folio_pos(folio), folio_size(folio),
 				   NETFS_READPAGE);
 	if (IS_ERR(rreq)) {
 		ret = PTR_ERR(rreq);
@@ -470,7 +470,7 @@ int netfs_write_begin(struct netfs_inode *ctx,
 	}
 
 	rreq = netfs_alloc_request(mapping, file,
-				   folio_file_pos(folio), folio_size(folio),
+				   folio_pos(folio), folio_size(folio),
 				   NETFS_READ_FOR_WRITE);
 	if (IS_ERR(rreq)) {
 		ret = PTR_ERR(rreq);

fs/netfs/buffered_write.c

@@ -54,7 +54,7 @@ static enum netfs_how_to_modify netfs_how_to_modify(struct netfs_inode *ctx,
 {
 	struct netfs_folio *finfo = netfs_folio_info(folio);
 	struct netfs_group *group = netfs_folio_group(folio);
-	loff_t pos = folio_file_pos(folio);
+	loff_t pos = folio_pos(folio);
 
 	_enter("");