In the Linux kernel, the following vulnerability has been resolved: net: phylink: correctly validate returned PCS in phylink_inband_caps I

In the Linux kernel, the following vulnerability has been resolved:

net: phylink: correctly validate returned PCS in phylink_inband_caps

In phylink_inband_caps(), the PCS returned by mac_select_pcs is only
checked if NULL but mac_select_pcs can also return an error pointer.

This can cause a kernel panic as phylink_pcs_inband_caps() only checks
if passed PCS is not NULL and directly dereference ops from the phylink_pcs
struct.

Use the IS_ERR_OR_NULL macro to address both case where the returned
PCS can be NULL or an error pointer and prevent a kernel panic.