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 <roland@purestorage.com>
Signed-off-by: Steve Hodgson <steve@purestorage.com>
Acked-by: Joern Engel <joern@logfs.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/btree.c

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