On exec(2), when loading the ELF image, pmap_enter_object() is
called to prefault pages. This is an obvious place for making
sure the I-cache is coherent. It was missing though. As such,
execution over NFS and ZFS file systems was failing. NFS was
fixed the wrong way (by flushing the D-cache as part of the
NFS code) in a previous commit. ZFS problems were encountered
after that and indicated that something else was wrong...

Approved by: re (kib)