rack: honor prior socket buffer lock when doing the upcall

While partially reverting D24237 with D29690, due to introducing some
unintended effects for in-kernel TCP consumers, the preexisting lock
on the socket send buffer was not considered properly.

Found by: markj
MFC after: 2 weeks
Reviewed By: tuexen, transport
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D30390