Fix two similar bugs in the populate vm_fault() code.

If pager' populate method succeeded, but other thread raced with us
and modified vm_map, we must unbusy all pages busied by the pager,
before we retry the whole fault handling. If pager instantiated more
pages than fit into the current map entry, we must unbusy the pages
which are clipped.

Also do some refactoring, clarify comments and use more clear local
variable names.

Reported and tested by: kargl, subbsd@gmail.com (previous version)
Reviewed by: alc
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks