__vdso_gettc(): be extra careful with /dev/hpet mappings, never unmap
the mapping which might be accessed by other threads.

If a pointer to the /dev/hpet register page mapping was stored into
the hpet_dev_map, other threads might access the page at any time.
Never unmap it, instead, keep track of mappings for all hpet units in
smal array. Store pointer to the newly mapped registers page using
CAS, to detect parallel mappings.

It appeared relatively easy to demonstrate the problem by arranging
two threads which perform gettimeofday(2) concurently, first time in
the process address space, when HPET is used for timecounter.

PR: 215715
Sponsored by: The FreeBSD Foundation
MFC after: 1 week