GFS2: constify attribute_group structures.

attribute_groups are not supposed to change at runtime. All functions
working with attribute_groups provided by <linux/sysfs.h> work with const
attribute_group. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   5259	   1344	      8	   6611	   19d3	fs/gfs2/sys.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   5371	   1216	      8	   6595	   19c3	fs/gfs2/sys.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>