gfs2: Follow-up to flag rename in sysfs status file

Follow up to commit e7beb8b6 ("gfs2: Rename SDF_DEACTIVATING to
SDF_KILL") and change the description of the SDF_KILL flag from
"Deactivating" to "Killing".

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>