x86/time: update vtsc_last with cmpxchg and drop vtsc_lock

x86/time: update vtsc_last with cmpxchg and drop vtsc_lock

Now that vtsc_last is the only entity protected by vtsc_lock we can
simply update it using a single atomic operation and drop the spinlock
entirely. This is extremely important for the case of running nested
(e.g. shim instance with lots of vCPUs assigned) since if preemption
happens somewhere inside the critical section that would immediately
mean that other vCPU stop progressing (and probably being preempted
as well) waiting for the spinlock to be freed.

This fixes constant shim guest boot lockups with ~32 vCPUs if there is
vCPU overcommit present (which increases the likelihood of preemption).

Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>