cxgbe: Remove most uses of sysctl_wire_old_buffer

Most of these sysctls don't call sbuf_* while holding any locks.  Of
the ones that do hold locks, all but one can be fixed to drop the lock
before calling sbuf_*.

Reviewed by:	np
Sponsored by:	Chelsio Communications
Differential Revision:	https://reviews.freebsd.org/D45186