sh: Kill off UTLB flush in fast-path.

The __do_page_fault() fast-path contains a UTLB flush in order to
force an ITLB reload, this isn't needed in practice as the ITLB is
auto-reloaded from the UTLB anyways, which is already displaced by
the manual 'ldtlb' in the update_mmu_cache() path.

This provides a measurable speed up in the TLB miss fast-path.

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