Make sure to free the sf_buf if we've allocated it but fail to allocate

an mbuf (ENOBUFS) before returning so that we don't leak sf_bufs in
the case where we're out of mbufs.

Submitted by: David Greenman (dg)

sys/kern/uipc_syscalls.c

@@ -1705,6 +1705,7 @@ retry_lookup:
 		MGETHDR(m, M_WAIT, MT_DATA);
 		if (m == NULL) {
 			error = ENOBUFS;
+			sf_buf_free((void *)sf->kva, PAGE_SIZE);
 			goto done;
 		}
 		/*