[GFS2] Make our address_space_operations const

As per Christoph's patch:
http://www.kernel.org/git/?p=linux/kernel/git/steve/gfs2-2.6.git;a=commitdiff;h=f5e54d6e53a20cef45af7499e86164f0e0d16bb2



We mark struct address_space_operations const in GFS2.

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