cxgbei: Explicitly clear the page pode reservation pointer after freeing it.

A single union ctl_io can be reused across multiple transfers (in
particular by the ramdisk backend).  On a reuse, the reservation
pointer would retain its value from the previous transfer tripping an
assertion.

Reported by:	Jithesh Arakkan @ Chelsio
Sponsored by:	Chelsio Communications

Differential Revision:	https://reviews.freebsd.org/D29939

sys/dev/cxgbe/cxgbei/icl_cxgbei.c

@@ -1162,6 +1162,7 @@ icl_cxgbei_conn_transfer_done(struct icl_conn *ic, void *arg)
 
 		t4_free_page_pods(prsv);
 		uma_zfree(prsv_zone, prsv);
+		io_to_ppod_reservation(ctsio) = NULL;
 	}
 }