fs/proc/mmu.c: headers butchery

fs/proc/mmu.c consists of only one function which uses only:
1) struct vmalloc_info *
2) struct vm_struct *
3) struct vmalloc_info
4) vmlist
5) VMALLOC_TOTAL, VMALLOC_START, VMALLOC_END
6) read_lock, read_unlock
7) vmlist_lock
8) struct vm_struct

This gives us linux/spinlock.h, asm/pgtable.h, "internal.h", linux/vmalloc.h.
asm/pgtable.h uses PKMAP_BASE on i386, for which asm/highmem.h is needed.
But, linux/highmem.h is actually used to make it compile everywhere.
I'll deal later with this particular i386 surprise.

Cross-compile tested on many archs and configs.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>