[LINUX][X86_64] Fix initial memory mapping code.

[LINUX][X86_64] Fix initial memory mapping code.

The temporary mappings needed to set up the 1:1 mappings must be torn down after use; otherwise they may trigger the
WARN_ON() in vmap_pte_range() (namely if the chunk allocated to hold kernel and initial page tables gets close to or
exceeds 128Mb, or if a sufficiently high mem= argument causes the static allocations to grow beyond 128Mb, which in
either case means these mappings extend into the modules area).

Signed-off-by: Jan Beulich <jbeulich@novell.com>