sh: Restore previous behaviour on kernel fault

The last commit changed the behaviour on kernel faults when we were
doing something other than syncing the page tables. vmalloc_sync_one()
needs to return NULL if the page tables are up to date, because the
reason for the fault was not a missing/inconsitent page table entry. By
returning NULL if the page tables are sync'd we signal to the calling
function that further work must be done to resolve this fault.

Also, remove the superfluous __va() around the first argument to
vmalloc_sync_one(). The value of pgd_k is already a virtual address and
using it wth __va() causes a NULL dereference.

Signed-off-by: Matt Fleming <matt@console-pimps.org>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>