swap_pager: fix assert in seek_data

An assertion that an object was write-locked should be instead an
assertion that the object is read locked.

Reported by:	Jenkins
Fixes:	 db08b0b tmpfs_vnops: move swap work to swap_pager
Differential Revision:	https://reviews.freebsd.org/D47278

sys/vm/swap_pager.c

@@ -2487,7 +2487,7 @@ swap_pager_seek_data(vm_object_t object, vm_pindex_t pindex)
 	vm_page_t m;
 	vm_pindex_t swap_index;
 
-	VM_OBJECT_ASSERT_WLOCKED(object);
+	VM_OBJECT_ASSERT_RLOCKED(object);
 	vm_page_iter_init(&pages, object);
 	m = vm_page_iter_lookup_ge(&pages, pindex);
 	if (m != NULL) {