xen/arm: Ensure the SSBD workaround is re-enabled right after exiting a guest

xen/arm: Ensure the SSBD workaround is re-enabled right after exiting a guest

At the moment, SSBD workaround is re-enabled for Xen after interrupts
are unmasked. This means we may end up to execute some part of the
hypervisor if an interrupt is received before the workaround is
re-enabled.

Each trap may require to unmask different interrupts.
As the rest of enter_hypervisor_from_guest() does not require to have
interrupts masked, the function is now split in two parts:
    1) enter_hypervisor_from_guest_preirq() called with interrupts
       masked.
    2) enter_hypervisor_from_guest() called with interrupts unmasked.

Note that while it might be possible to avoid spliting the function in
two parts, it requires a bit more work than I can currently invest to
avoid using indirect branch.

Furthermore, the function name is rather generic as there might be more
work to dob before interrupts are unmasked in the future.

Fixes: a7898e4c59 ("xen/arm: Add ARCH_WORKAROUND_2 support for guests")
Reported-by: Andrii Anisov <andrii_anisov@epam.com>
Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Release-acked-by: Juergen Gross <jgross@suse.com>