Use the flush function introduced with the POWER9 machine check handler
for POWER7 and 8, rather than open coding it multiple times in callers.

There is a specific ERAT flush type introduced for POWER9, but the
POWER7-8 ERAT errors continue to do SLB flushing (which also flushes
ERAT), so as not to introduce functional changes with this cleanup
patch.
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>