xen: make HYPERVISOR_set_debugreg() always_inline

xen: make HYPERVISOR_set_debugreg() always_inline

HYPERVISOR_set_debugreg() is being called from noinstr code, so it
should be attributed "always_inline".

Fixes: 50ee3dd624e9e3a8df2 ("x86/xen: Make set_debugreg() noinstr")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/20211125092056.24758-3-jgross@suse.com
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>