sh: Handle a NULL vma in __update_tlb() for the fast-path.

sh: Handle a NULL vma in __update_tlb() for the fast-path.

The TLB miss fast-path presently calls in to update_mmu_cache() to
set up the entry, and does so with a NULL vma. Check for vma validity
in the __update_tlb() ptrace checks.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>