cxgbe(4): Stop but don't free netmap queues when netmap is switched off.

It is common for freelists to be starving when a netmap application
stops. Mailbox commands to free queues can hang in such a situation.
Avoid that by not freeing the queues when netmap is switched off.
Instead, use an alternate method to stop the queues without releasing
the context ids. If netmap is enabled again later then the same queue
is reinitialized for use. Move alloc_nm_rxq and txq to t4_netmap.c
while here.

MFC after: 1 week
Sponsored by: Chelsio Communications