ceph: fix num_pages_free accounting in pagelist

Decrement the free page counter when removing a page from the free_list.

Signed-off-by: Sage Weil <[e-mail]>

net/ceph/pagelist.c

···
 	} else {
 		page = list_first_entry(&pl->free_list, struct page, lru);
 		list_del(&page->lru);
+		--pl->num_pages_free;
 	}
 	if (!page)
 		return -ENOMEM;