reiserfscore: make reiserfs_open's filename argument const

Signed-off-by: Jeff Mahoney <jeffm@suse.com>

include/reiserfs_lib.h

@@ -75,7 +75,7 @@ typedef struct _transaction reiserfs_trans_t;
 void init_tb_struct(struct tree_balance *tb, reiserfs_filsys_t *,
 		    struct reiserfs_path *path, int size);
 
-reiserfs_filsys_t *reiserfs_open(char *filename, int flags, int *error,
+reiserfs_filsys_t *reiserfs_open(const char *filename, int flags, int *error,
 				 void *vp, int skip_check);
 reiserfs_filsys_t *reiserfs_create(char *filename, int version,
 				   unsigned long block_count, int block_size,