zed crashes when devid not present

zed core dumps due to a NULL pointer in zfs_agent_iter_vdev(). The
gs_devid is NULL, but the nvl has a "devid" entry.

zfs_agent_post_event() checks that ZFS_EV_VDEV_GUID or DEV_IDENTIFIER is
present in nvl, but then later it and zfs_agent_iter_vdev() assume that
DEV_IDENTIFIER is present and thus gs_devid is set.

Typically this is not a problem because usually either all vdevs have
devid's, or none of them do. Since zfs_agent_iter_vdev() first checks if
the vdev has devid before dereferencing gs_devid, the problem isn't
typically encountered. However, if some vdevs have devid's and some do
not, then the problem is easily reproduced. This can happen if the pool
has been moved from a system that has devid's to one that does not.

The fix is for zfs_agent_iter_vdev() to only try to match the devid's if
both nvl and gsp have devid's present.

Reviewed-by: Prashanth Sreenivasa <pks@delphix.com>
Reviewed-by: Don Brady <don.brady@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: loli10K <ezomori.nozomu@gmail.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
External-issue: DLPX-65090
Closes #9054
Closes #9060