reiserfs: fix corruption introduced by balance_leaf refactor

Commits f1f007c3 (reiserfs: balance_leaf refactor, pull out
balance_leaf_insert_left) and cf22df18 (reiserfs: balance_leaf
refactor, pull out balance_leaf_paste_left) missed that the `body'
pointer was getting repositioned. Subsequent users of the pointer
would expect it to be repositioned, and as a result, parts of the
tree would get overwritten. The most common observed corruption
is indirect block pointers being overwritten.

Since the body value isn't actually used anymore in the called routines,
we can pass back the offset it should be shifted. We constify the body
and ih pointers in the balance_leaf as a mostly-free preventative measure.

Cc: <stable@vger.kernel.org> # 3.16
Reported-and-tested-by: Jeff Chua <jeff.chua.linux@gmail.com>
Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/reiserfs/lbalance.c

@@ -899,8 +899,9 @@ void leaf_delete_items(struct buffer_info *cur_bi, int last_first,
 
 /* insert item into the leaf node in position before */
 void leaf_insert_into_buf(struct buffer_info *bi, int before,
-			  struct item_head *inserted_item_ih,
-			  const char *inserted_item_body, int zeros_number)
+			  struct item_head * const inserted_item_ih,
+			  const char * const inserted_item_body,
+			  int zeros_number)
 {
 	struct buffer_head *bh = bi->bi_bh;
 	int nr, free_space;

fs/reiserfs/reiserfs.h

@@ -3216,11 +3216,12 @@ int leaf_shift_right(struct tree_balance *tb, int shift_num, int shift_bytes);
 void leaf_delete_items(struct buffer_info *cur_bi, int last_first, int first,
 		       int del_num, int del_bytes);
 void leaf_insert_into_buf(struct buffer_info *bi, int before,
-			  struct item_head *inserted_item_ih,
-			  const char *inserted_item_body, int zeros_number);
-void leaf_paste_in_buffer(struct buffer_info *bi, int pasted_item_num,
-			  int pos_in_item, int paste_size, const char *body,
+			  struct item_head * const inserted_item_ih,
+			  const char * const inserted_item_body,
 			  int zeros_number);
+void leaf_paste_in_buffer(struct buffer_info *bi, int pasted_item_num,
+			  int pos_in_item, int paste_size,
+			  const char * const body, int zeros_number);
 void leaf_cut_from_buffer(struct buffer_info *bi, int cut_item_num,
 			  int pos_in_item, int cut_size);
 void leaf_paste_entries(struct buffer_info *bi, int item_num, int before,