[GFS2] fix bz 231369, gfs2 will oops if you specify an invalid mount option

If you specify an invalid mount option when trying to mount a gfs2 filesystem,
gfs2 will oops.  The attached patch resolves this problem.

Signed-off-by: Josef Whiter <jwhiter@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>