pstore: constify pstore_zbackend structures

The references of pstore_zbackend structures are stored into the
pointer zbackend of type struct pstore_zbackend. The pointer zbackend
can be made const as it is only dereferenced. After making this change
the pstore_zbackend structures whose references are stored into the
pointer zbackend can be made const too.

File size before:
   text	   data	    bss	    dec	    hex	filename
   4817	    541	    172	   5530	   159a	fs/pstore/platform.o

File size after:
   text	   data	    bss	    dec	    hex	filename
   4865	    477	    172	   5514	   158a	fs/pstore/platform.o

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Kees Cook <keescook@chromium.org>