Use a dedicated counter for inactive queue scans.

The laundry thread keeps track of the number of inactive queue scans
performed by the page daemon, and was previously using the v_pdwakeups
counter to count them. However, in some cases the inactive queue may
be scanned multiple times after a single wakeup, so it's more accurate
to use a dedicated counter.

Reviewed by: alc, kib (previous version)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D13422