[GFS2] Mark file_operations const

As per Arjan's patches:
http://www.kernel.org/git/?p=linux/kernel/git/steve/gfs2-2.6.git;a=commitdiff;h=99ac48f54a91d02140c497edc31dc57d4bc5c85d
and
http://www.kernel.org/git/?p=linux/kernel/git/steve/gfs2-2.6.git;a=commitdiff;h=4b6f5d20b04dcbc3d888555522b90ba6d36c4106



make the GFS2 file_operations structures const.

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