The following patch is required so that munmap(0x8000, *) does not cause
ARM kernels to crash.  The problem is that the machine vectors are at
virtual address 0.  Unfortunately, when free_pgtables() is called, it
clears first level page tables starting at 0 in this case, and takes
out the machine vectors.  This then results in an unrecoverable hang.

We already have FIRST_USER_PGD_NR to define the first entry in the pgd
which may be cleared, so we use this to clamp "start_index"
appropriately.  The following patch does this.  Tested on ARM.