x86-64: don't check for map replacement

The check prevents flags on mappings from being changed, which is not
desireable.  There's no need to check for replacing a mapping, and
x86-32 does not do this check.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>