powerpc/mm: Add tracking of the number of coprocessors using a context

Currently, when using coprocessors (which use the Nest MMU), we
simply increment the active_cpu count to force all TLB invalidations
to be come broadcast.

Unfortunately, due to an errata in POWER9, we will need to know
more specifically that coprocessors are in use.

This maintains a separate copros counter in the MMU context for
that purpose.

NB. The commit mentioned in the fixes tag below is not at fault for
the bug we're fixing in this commit and the next, but this fix applies
on top the infrastructure it introduced.

Fixes: 03b8abed ("cxl: Enable global TLBIs for cxl contexts")
Cc: stable@vger.kernel.org # v4.15+
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Tested-by: Balbir Singh <bsingharora@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>