EFI: map allocation size must be set to zero

EFI: map allocation size must be set to zero

Commit 8a753b3f1c ("efi: fix allocation problems if ExitBootServices()
fails") replaced the use of a static (and hence zero-initialized)
variable by an automatic (and hence uninitialized) one.

Also drop the variable introduced by that commit in favor of re-using
another available and suitable one.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Ross Lagerwall <ross.lagerwall@citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>