Revert "xen/balloon: Fix crash when ballooning on x86 32 bit PAE"

This reverts commit dfd74a1e.

This has been fixed by commit dca4436d

 which added the out
of bounds check to __add_memory, so that trying to add blocks past
MAX_PHYSMEM_BITS will fail.

Note the check in the Xen balloon driver was bogus anyway, as it
checked the start address of the resource, but it should instead test
the end address to assert the whole resource falls below
MAX_PHYSMEM_BITS.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20200727091342.52325-4-roger.pau@citrix.com
Signed-off-by: Juergen Gross <jgross@suse.com>