x86: move __VMALLOC_RESERVE to pgtable_32.c

Impact: cleanup

The __VMALLOC_RESERVE global variable is not used in init_32.c. Move that to
pgtable_32.c to reduce the diff between init_32.c and init_64.c.

Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>
LKML-Reference: <1236077704.2675.4.camel@penberg-laptop>
Signed-off-by: Ingo Molnar <mingo@elte.hu>