x86-64: reduce boot fixmap space

Just like for 32-bit - as 256 entries are needed, aligning to a
256-entry boundary is sufficient and still guarantees the single pte
table requirement.

Likewise move up __end_of_permanent_fixed_addresses, to match 32-bit.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>