chtls: Fix potential resource leak

The dst entry should be released if no neighbour is found. Goto label
free_dst to fix the issue. Besides, the check of ndev against NULL is
redundant.

Signed-off-by: Pan Bian <bianpan2016@163.com>
Link: https://lore.kernel.org/r/20210121145738.51091-1-bianpan2016@163.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>