Add missing MMP status code to libzfs_status

When MMP was merged the status codes in libzfs_status were not
updated to add the status code for ZPOOL_STATUS_IO_FAILURE_MMP. This
commit corrects this and adds comments to help keep track of which
code is used for which status.

Reviewed-by: Olaf Faaland <faaland1@llnl.gov>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: bunder2015 <omfgbunder@gmail.com>
Closes #8148
Closes #8222