powerpc: Handle machine checks caused by D-ERAT multihit

Instead of crashing the user process when a D-ERAT multihit is detected, try
to flush the ERAT, and continue. This machine check indicates a likely PMAP
invalidation shortcoming that will need to be addressed, but it's
recoverable, so just recover. The recovery is pmap-specific to flush the
ERAT, so add a pmap function to do so, currently only implemented by the
POWER9 radix pmap.