Do not try to fill socket send buffer to the last byte.

Setting so_snd.sb_lowat to at least 1/8 of the socket buffer size allows
send thread more actively use PDUs coalescing, that dramatically reduces
TCP lock congestion and number of context switches, when the socket is
full and PDUs are small.

MFC after:	1 week
Sponsored by:	iXsystems, Inc.

sys/dev/iscsi/icl_soft.c

@@ -914,7 +914,8 @@ icl_conn_send_pdus(struct icl_conn *ic, struct icl_pdu_stailq *queue)
 				    "have %ld, need %ld",
 				    available, size);
 #endif
-				so->so_snd.sb_lowat = size;
+				so->so_snd.sb_lowat = max(size,
+				    so->so_snd.sb_hiwat / 8);
 				SOCKBUF_UNLOCK(&so->so_snd);
 				return;
 			}