GFS2: write_end error path fails to unlock transaction lock

I did an audit of gfs2's transaction glock for bugzilla bug
658619 and ran across this:

In function gfs2_write_end, in the unlikely event that
gfs2_meta_inode_buffer returns an error, the code may forget
to unlock the transaction lock because the "failed" label
appears after the call to function gfs2_trans_end.

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