MIPS: tlb-r3k: Optimise a TLBWI barrier in TLB invalidation

Replace an explicit barrier with a useful processor instruction in TLB
invalidation, following several other such cases elsewhere in
`tlb-r3k.c'.

Signed-off-by: Maciej W. Rozycki <macro@linux-mips.org>
Cc: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/10196/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>