Only return the current cpu if it's in the cpumask. When we restrict the
cpumask it probably means we are unable to sent interrupts to CPUs outside
the map. As such only return the current CPU when it's within the mask
otherwise return the first valid CPU.

This is needed on ThunderX as, in a dual socket configuration, we are
unable to send MSI/MSI-X interrupts between sockets.

Reviewed by: mmel
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D11957