btree: export btree_get_prev() so modules can use btree_for_each

The btree_for_each API is implemented with macros that internally call
btree_get_prev(), so if btree_get_prev() isn't exported then modules fail
to link if they try to use one of the btree_for_each macros.  Since the
rest of the btree API is exported, we should keep things orthogonal and
make this work too.

Signed-off-by: Roland Dreier <[email protected]>
Signed-off-by: Steve Hodgson <[email protected]>
Acked-by: Joern Engel <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

lib/btree.c

@@ -357,6 +357,7 @@ void *btree_get_prev(struct btree_head *head, struct btree_geo *geo,
 	}
 	return NULL;
 }
+EXPORT_SYMBOL_GPL(btree_get_prev);
 
 static int getpos(struct btree_geo *geo, unsigned long *node,
 		unsigned long *key)