cifs: Change from mempool_destroy to mempool_exit for request pools

insmod followed by rmmod was oopsing with the new mempools cifs request patch

Fixes: edea94a ("cifs: Add mempools for cifs_io_request and cifs_io_subrequest structs")
Suggested-by: David Howells <dhowells@redhat.com>
Reviewed-by: Enzo Matsumiya <ematsumiya@suse.de>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: Steve French

fs/smb/client/cifsfs.c

@@ -1790,9 +1790,9 @@ static int cifs_init_netfs(void)
 
 static void cifs_destroy_netfs(void)
 {
-	mempool_destroy(&cifs_io_subrequest_pool);
+	mempool_exit(&cifs_io_subrequest_pool);
 	kmem_cache_destroy(cifs_io_subrequest_cachep);
-	mempool_destroy(&cifs_io_request_pool);
+	mempool_exit(&cifs_io_request_pool);
 	kmem_cache_destroy(cifs_io_request_cachep);
 }