if_ntb: Transport link cleanup needs to be on a taskqueue

Because it can sleep drainking link work callout(s). Linux (dual
BSD/GPL driver) does something very similar.

At the same time, switch the NTB CTX lock to a non-spin mutex, because
the taskqueue_swi lock can't be taken after a spin mutex.

Suggested by: Witness
Sponsored by: EMC / Isilon Storage Division