GFS2: Fix permissions checking for setflags ioctl()

CVE-2010-1641

We should be checking for the ownership of the file for which
flags are being set, rather than just for write access.

Reported-by: Dan Rosenberg <dan.j.rosenberg@gmail.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>
(cherry-picked from commit 7df0e039

 upstream)
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>