eCryptfs: 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
   6122	    636	     24	   6782	   1a7e	fs/ecryptfs/main.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   6186	    604	     24	   6814	   1a9e	fs/ecryptfs/main.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>