staging: lustre: 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
   9489	    992	     40	  10521	   2919	lustre/lustre/osc/lproc_osc.o
   1289	    288	      0	   1577	    629	lustre/lustre/lmv/lproc_lmv.o
   3794	    928	     40	   4762	   129a	lustre/lustre/lov/lproc_lov.o
   3802	    576	     40	   4418	   1142	lustre/lustre/mdc/lproc_mdc.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   9553	    928	     40	  10521	   2919	lustre/lustre/osc/lproc_osc.o
   1353	    224	      0	   1577	    629	lustre/lustre/lmv/lproc_lmv.o
   3858	    864	     40	   4762	   129a	lustre/lustre/lov/lproc_lov.o
   3866	    512	     40	   4418	   1142	lustre/lustre/mdc/lproc_mdc.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>