Revert "x86_64: there's no need to preallocate level1_fixmap_pgt"

This reverts commit 033786969d1d1b5af12a32a19d3a760314d05329.

Suresh Siddha reported that this broke booting on his 2GB testbox.

Reported-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>