[netmap] fix locking regressions

• Firmware oriented NICs may need to sleep in their configuration paths. Use RLOCK instead of WLOCK to allow this to again occur.
  
  This fixes netmap on cxgbe.

• Change the worker lock to a normal mutex rather than a spin lock. Drivers shouldn't be doing netmap work from the fast interrupt handlers, so it's not required to be a spinlock.

Submitted by: luigi, Vincenzo Maffione <v.maffione@gmail.com>
Reviewed by: jhb