The count argument natural type if vm_pindex_t, but due to the loop
organization, it has to be signed type to detect the termination
condition. Replace this logic by using distinguished counter for the
processed pages, and terminate loop when the counter exceeds the
argument.
Completely process one swblock for all relevant indexes instead of
doing relookup in hash when incrementing page index on the loop step.
Do not drop hash mutex around iterations.
Noted and reviewed by: alc
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
da005e5578