iflib: Avoid locking for unsupported VF status queries

ifconfig -v requests SR-IOV VF status from every interface. iflib
previously acquired the context lock before dispatching the request even
for VFs and drivers using the default unsupported method. Mailbox work
on a VF could therefore delay the complete interface listing.

VF status describes the children of an SR-IOV PF. Reject requests on
VF contexts and classes using the default method without taking the
context lock. Keep the lock for actual PF status providers.

Fixes: 1ccf543b21ef ("ifconfig: Add SR-IOV VF status output")