xen/efi: Set nonblocking callbacks

[ Upstream commit df359f0d09dc029829b66322707a2f558cb720f7 ]

Other parts of the kernel expect these nonblocking EFI callbacks to
exist and crash when running under Xen. Since the implementations of
xen_efi_set_variable() and xen_efi_query_variable_info() do not take any
locks, use them for the nonblocking callbacks too.

Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>