Properly ignore PDUs with CmdSN outside of allowed range.

Approved by:	re (glebius)
Sponsored by:	FreeBSD Foundation
In pmap_clear_modify(), initialize pvh even for fictitious managed
page, otherwise the small mappings loop would use uninitialized value.
Note that currently pmap_clear_modify() is not called for fictitious
pages.

Sponsored by:	The FreeBSD Foundation
Approved by:	re (glebius)