[GFS2] Initialize extent_list earlier

Here is a patch for the latest upstream GFS2 code:
The journal extent map needs to be initialized sooner than it
currently is.  Otherwise failed mount attempts (e.g. not enough
journals, etc.) may panic trying to access the uninitialized list.

Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>