kexec: fix some types

kexec: fix some types

crashinfo_maxaddr can/will be a 64-bit type, hence passing it to fls()
is wrong.

crashinfo_maxaddr_bits, otoh, doesn't need to be a paddr_t, and can
additionally be __initdata.

struct xen_kexec_reserve's start field - representing a physical
address - ought to have respective type.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>