pstore: Introduce get_reason_str() to pstore

pstore: Introduce get_reason_str() to pstore

Recently, there has been some changes in kmsg_dump() below and they have been applied to linus-tree.
 (1) kmsg_dump(KMSG_DUMP_KEXEC) was removed.
     http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=7bd6057a4acee9257409eae7536c85d600762e5e

 (2) The order of "enum kmsg_dump_reason" was modified.
     http://git.kernel.org/?p=linux/kernel/git/torvalds/linux.git;a=commitdiff;h=bd88934494fe048bb2dd605506795a7c6705e751

Replace the fragile reason_str array with a more robust solution that
will not be broken by future re-arrangements of the enum values.

Signed-off-by: Seiji Aguchi <seiji.aguchi@hds.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Acked-by: Don Zickus <dzickus@redhat.com>
Link: https://lkml.org/lkml/2012/3/16/417
Signed-off-by: Tony Luck <tony.luck@intel.com>