powerpc/mm/radix: Improve TLB/PWC flushes

At the moment we have to rather sub-optimal flushing behaviours:

 - flush_tlb_mm() will flush the PWC which is unnecessary (for example
   when doing a fork)

 - A large unmap will call flush_tlb_pwc() multiple times causing us
   to perform that fairly expensive operation repeatedly. This happens
   often in batches of 3 on every new process.

So we change flush_tlb_mm() to only flush the TLB, and we use the
existing "need_flush_all" flag in struct mmu_gather to indicate
that the PWC needs flushing.

Unfortunately, flush_tlb_range() still needs to do a full flush
for now as it's used by the THP collapsing. We will fix that later.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Reviewed-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>