qlcnic: prevent ->dcb use-after-free on qlcnic_dcb_enable() failure

[ Upstream commit 13a7c896 ]

adapter->dcb would get silently freed inside qlcnic_dcb_enable() in
case qlcnic_dcb_attach() would return an error, which always happens
under OOM conditions. This would lead to use-after-free because both
of the existing callers invoke qlcnic_dcb_get_info() on the obtained
pointer, which is potentially freed at that point.

Propagate errors from qlcnic_dcb_enable(), and instead free the dcb
pointer at callsite using qlcnic_dcb_free(). This also removes the now
unused qlcnic_clear_dcb_ops() helper, which was a simple wrapper around
kfree() also causing memory leaks for partially initialized dcb.

Found by Linux Verification Center (linuxtesting.org) with the SVACE
static analysis tool.

Fixes: 3c44bba1

 ("qlcnic: Disable DCB operations from SR-IOV VFs")
Reviewed-by: Michal Swiatkowski <michal.swiatkowski@linux.intel.com>
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>