vm_domainset: Print correct function in KASSERT()/panic()

Some messages in vm_domainset_iter_next() would wrongly refer to
vm_domainset_iter_first(). While here, ensure that all assertion/panic
messages use 'func' to avoid this discrepancy in the future if code
is moved/copy-pasted again.

Reviewed by: markj, alc, kib
MFC after: 10 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51248