gfs2: simplify gfs2_freeze by removing case

Function gfs2_freeze had a case statement that simply checked the
error code, but the break statements just made the logic hard to
read. This patch simplifies the logic in favor of a simple if.

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