Fix EINVAL message when CPU binding information is requested for IRQ.

`cpuset -g -x N` along with requested information always prints
message `cpuset: getdomain: Invalid argument'. The EINVAL is returned
from kern_cpuset_getdomain(), since it doesn't expect CPU_LEVEL_WHICH
and CPU_WHICH_IRQ parameters.

To fix the error, do not call cpuset_getdomain() when `-x' is specified.