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>

fs/pstore/platform.c

@@ -267,7 +267,7 @@ static void free_zlib(void)
 	big_oops_buf_sz = 0;
 }
 
-static struct pstore_zbackend backend_zlib = {
+static const struct pstore_zbackend backend_zlib = {
 	.compress	= compress_zlib,
 	.decompress	= decompress_zlib,
 	.allocate	= allocate_zlib,
@@ -328,7 +328,7 @@ static void free_lzo(void)
 	big_oops_buf_sz = 0;
 }
 
-static struct pstore_zbackend backend_lzo = {
+static const struct pstore_zbackend backend_lzo = {
 	.compress	= compress_lzo,
 	.decompress	= decompress_lzo,
 	.allocate	= allocate_lzo,
@@ -393,7 +393,7 @@ static void free_lz4(void)
 	big_oops_buf_sz = 0;
 }
 
-static struct pstore_zbackend backend_lz4 = {
+static const struct pstore_zbackend backend_lz4 = {
 	.compress	= compress_lz4,
 	.decompress	= decompress_lz4,
 	.allocate	= allocate_lz4,
@@ -402,7 +402,7 @@ static struct pstore_zbackend backend_lz4 = {
 };
 #endif
 
-static struct pstore_zbackend *zbackend =
+static const struct pstore_zbackend *zbackend =
 #if defined(CONFIG_PSTORE_ZLIB_COMPRESS)
 	&backend_zlib;
 #elif defined(CONFIG_PSTORE_LZO_COMPRESS)