[GFS2] remove gfs2_dev_iops

struct inode_operations gfs2_dev_iops is always the same as gfs2_file_iops,
since Jan 2006, when GFS2 merged into mainstream kernel.

So one of them could be removed.

Signed-off-by: Denis Cheng <crquan@gmail.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>