gfs2: Prevent double iput for journal on error

When a gfs2 file system is withdrawn it does iput on its journal to
allow recovery from another cluster node. If it's unable to get a
replacement inode for whatever reason, the journal descriptor would
still be pointing at the evicted inode. So when unmount clears out the
list of journals, it would do a second iput referencing the pointer.
To avoid this, set the inode pointer to NULL.

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