radix_tree.h trivial comment correction

There is an unmatched parenthesis in the locking commentary of radix_tree.h
which is trivially fixed by the patch below.

Signed-off-by: Tim Pepper <lnxninja@linux.vnet.ibm.com>
Acked-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Adrian Bunk <bunk@kernel.org>