gfs2: Rename remaining "transaction" glock references

The transaction glock was repurposed to serve as the new freeze glock
years ago.  Don't refer to it as the transaction glock anymore.

Also, to be more precise, call it the "freeze glock" instead of the
"freeze lock".  Ditto for the journal glock.

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