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

(cherry picked from commit 61089df8147eb7109696476c891514296d543bad)