[tcp] Keep socket buffer locked until upcall

r367492 would unlock the socket buffer before eventually calling the upcall.
This leads to problematic interaction with NFS kernel server/client components
(MP threads) accessing the socket buffer with potentially not correctly updated
state.

Reported by: rmacklem
Reviewed By: tuexen, transport
Tested by: rmacklem, otis
MFC after: 2 weeks
Sponsored By: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D29690

(cherry picked from commit 032bf749fd44ac5ff20aab2c3d8e3c05491778ea)