Fix a few more places to handle ofld tx queues for RATELIMIT.

• Drain offload transmit queues when RATELIMIT is enabled but TCP_OFFLOAD is not.
• Expose the per-VI nofldtxq and first_ofld_txq sysctls when RATELIMIT is enabled but TCP_OFFLOAD is not.
• Clear offload transmit queue stats as part of a 'cxgbetool clearstats' request when RATELIMIT is enabled but TCP_OFFLOAD is not.

Reviewed by: np
MFC after: 2 weeks
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D18966