CVE-2021-47556

In the Linux kernel, the following vulnerability has been resolved:
ethtool: ioctl: fix potential NULL deref in ethtool_set_coalesce()
ethtool_set_coalesce() now uses both the .get_coalesce() and
.set_coalesce() callbacks. But the check for their availability is buggy,
so changing the coalesce settings on a device where the driver provides
only one of the callbacks results in a NULL pointer dereference instead
of an -EOPNOTSUPP. Fix the condition so that the availability of both
callbacks is ensured. This also matches the netlink code. Note that
reproducing this requires some effort - it only affects the legacy ioctl
path, and needs a specific combination of driver options: - have
.get_coalesce() and .coalesce_supported but no .set_coalesce(), or - have
.set_coalesce() but no .get_coalesce(). Here eg. ethtool doesn’t cause the
crash as it first attempts to call ethtool_get_coalesce() and bails out on
error.