GFS2: Fix error path ref counting for root inode

We were keeping hold of an extra ref to the root inode in one
of the error paths, that resulted in a hang.

Reported-by: Nate Straz <nstraz@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
Tested-by: Robert Peterson <rpeterso@redhat.com>