Do a more targeted check on the page cache and avoid to check the cache

pointer directly in vnode_pager_setsize() by using newly introduced
vm_page_is_cached() function.

Reviewed by:	alc
MFC after:	2 weeks
X-MFC:		r234039,234064

sys/vm/vnode_pager.c

@@ -440,7 +440,7 @@ vnode_pager_setsize(vp, nsize)
 			 */
 			vm_page_clear_dirty(m, base, PAGE_SIZE - base);
 		} else if ((nsize & PAGE_MASK) &&
-		    __predict_false(object->cache != NULL)) {
+		    vm_page_is_cached(object, OFF_TO_IDX(nsize))) {
 			vm_page_cache_free(object, OFF_TO_IDX(nsize),
 			    nobjsize);
 		}