Fix an issue in r338862.

For pmap_invalidate_all_pcid(), only reset pm_gen for non-kernel
pmaps, as it was done before the conversion to ifuncs. The reset is
useless but innocent for kernel_pmap. Coverity reported that cpuid is
used uninitialized in this case.

Reported by: cem
Reviewed by: alc, cem, markj
CID: 1395807
Sponsored by: The FreeBSD Foundation
Approved by: re (gjb)
Differential revision: https://reviews.freebsd.org/D17314