Fix mpr(4) panics caused by bad drive mapping tables

sys/dev/mpr/mpr_mapping.c
If _mapping_process_dpm_pg0 detects inconsistencies in the drive
mapping table (stored in the HBA's NVRAM), abort reading it and
continue to boot as if the mapping table were blank. I observed
such inconsistencies in several HBAs after upgrading firmware from
14.0.0.0 to 15.0.0.0.

Reviewed by: slm
MFC after: 3 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D12901