x86/mm: don't check alloc_boot_pages return

x86/mm: don't check alloc_boot_pages return

The only way alloc_boot_pages will return 0 is during the error case.
Although, Xen will panic in the error path. So the check in the caller
is pointless.

Looking at the loop, my understanding is it will try to allocate in
smaller chunk if a bigger chunk fail. Given that alloc_boot_pages can
never check, the loop seems unecessary.

Signed-off-by: Julien Grall <julien.grall@arm.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>