netfs: Use folio_next_pos()

This is one instruction more efficient than open-coding folio_pos() +
folio_size().  It's the equivalent of (x + y) << z rather than
x << z + y << z.

Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Link: https://patch.msgid.link/20251024170822.1427218-9-willy@infradead.org
Acked-by: David Howells <dhowells@redhat.com>
Reviewed-by: Paulo Alcantara (Red Hat) <pc@manguebit.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Paulo Alcantara <pc@manguebit.org>
Cc: netfs@lists.linux.dev
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: Matthew Wilcox (Oracle)

fs/netfs/buffered_write.c

@@ -535,7 +535,7 @@ vm_fault_t netfs_page_mkwrite(struct vm_fault *vmf, struct netfs_group *netfs_gr
 		folio_unlock(folio);
 		err = filemap_fdatawrite_range(mapping,
 					       folio_pos(folio),
-					       folio_pos(folio) + folio_size(folio));
+					       folio_next_pos(folio));
 		switch (err) {
 		case 0:
 			ret = VM_FAULT_RETRY;

fs/netfs/misc.c

@@ -298,7 +298,7 @@ bool netfs_release_folio(struct folio *folio, gfp_t gfp)
 	if (folio_test_dirty(folio))
 		return false;
 
-	end = umin(folio_pos(folio) + folio_size(folio), i_size_read(&ctx->inode));
+	end = umin(folio_next_pos(folio), i_size_read(&ctx->inode));
 	if (end > ctx->zero_point)
 		ctx->zero_point = end;