sockets: remove the socket-on-stack hack from sorflush()

The hack can be tracked down to 4.4BSD, where copy was performed
under splimp() and then after splx() dom_dispose was called.
Stevens has a chapter on this function, but he doesn't answer why
this trick is necessary. Why can't we call into dom_dispose under
splimp()? Anyway, with multithreaded kernel the hack seems to be
necessary to avoid LORs between socket buffer lock and different
filesystem locks, especially network file systems.

The new socket buffers KPI sbcut() from 1d2df300e9b allow us to get
rid of the hack.

Reviewed by: markj
Differential revision: https://reviews.freebsd.org/D35125