x86/xen: Tolerate nested XEN_LAZY_MMU entering/leaving

x86/xen: Tolerate nested XEN_LAZY_MMU entering/leaving

With the support of nested lazy mmu sections it can happen that
arch_enter_lazy_mmu_mode() is being called twice without a call of
arch_leave_lazy_mmu_mode() in between, as the lazy_mmu_*() helpers
are not disabling preemption when checking for nested lazy mmu
sections.

This is a problem when running as a Xen PV guest, as
xen_enter_lazy_mmu() and xen_leave_lazy_mmu() don't tolerate this
case.

Fix that in xen_enter_lazy_mmu() and xen_leave_lazy_mmu() in order
not to hurt all other lazy mmu mode users.

Fixes: 291b3abed657 ("x86/xen: use lazy_mmu_state when context-switching")
Tested-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Message-ID: <20260508143933.493013-1-jgross@suse.com>