GFS2: GFS2 will panic if you misspell any mount options

The gfs2 superblock pointer is NULL after a failed mount. When control
eventually goes to gfs2_kill_sb, we dereference this NULL pointer. This
patch ensures that the gfs2 superblock pointer is not NULL before being
dereferenced in gfs2_kill_sb.

Signed-off-by: Abhijith Das <adas@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>