x86/mm: Add global ASID process exit helpers

A global ASID is allocated for the lifetime of a process. Free the global ASID
at process exit time.

  [ bp: Massage, create helpers, hide details inside them. ]

Signed-off-by: Rik van Riel <riel@surriel.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250226030129.530345-10-riel@surriel.com