net: hwbm: Fix unbalanced spinlock in error case

When hwbm_pool_add exited in error the spinlock was not released. This
patch fixes this issue.

Fixes: 8cb2d8bf

 ("net: add a hardware buffer management helper API")
Reported-by: Jean-Jacques Hiblot <jjhiblot@traphandler.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>