pstore: cleanups to pstore_dump()

pstore_dump() can be called with many different "reason" codes. Save
the name of the code in the persistent store record.

Also - only worthwhile calling pstore_mkfile for KMSG_DUMP_OOPS - that
is the only one where the kernel will continue running.

Reviewed-by: Seiji Aguchi <seiji.aguchi@hds.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>