befs: constify stuff a bit

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/befs/btree.h b/fs/befs/btree.h
index 92e781e..f2a8f63 100644
--- a/fs/befs/btree.h
+++ b/fs/befs/btree.h
@@ -4,10 +4,10 @@
  */
 
 
-int befs_btree_find(struct super_block *sb, befs_data_stream * ds,
+int befs_btree_find(struct super_block *sb, const befs_data_stream *ds,
 		    const char *key, befs_off_t * value);
 
-int befs_btree_read(struct super_block *sb, befs_data_stream * ds,
+int befs_btree_read(struct super_block *sb, const befs_data_stream *ds,
 		    loff_t key_no, size_t bufsize, char *keybuf,
 		    size_t * keysize, befs_off_t * value);