In tcp_usr_send() if the pcb appears to be dropped, avoid freeing
M_NOTREADY mbufs.

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Revert M_SBCUT changes, the flag isn't actually needed. If sf_iodone()
does soisdisconnected() in case of I/O error, then PCB would be marked
dropped and if any more I/Os are pending on this socket, they will free
their mbufs in tcp_usr_ready().

Sponsored by:	Netflix
Sponsored by:	Nginx, Inc.
Set B_ASYNC on asynchronous buf.

Submitted by:	kib