Description: net: dsa: fix a crash if ->get_sset_count() fails
References:
Notes:
 carnil> Introduced in badf3ada60ab ("net: dsa: Provide CPU port statistics to master
 carnil> netdev"). Vulnerable versions: 4.7-rc1.
Bugs:
upstream: released (5.13-rc4) [a269333fa5c0c8e53c92b5a28a6076a28cde3e83]
6.7-upstream-stable: N/A "Fixed before branching point"
6.6-upstream-stable: N/A "Fixed before branching point"
6.1-upstream-stable: N/A "Fixed before branching point"
5.10-upstream-stable: released (5.10.42) [caff86f85512b8e0d9830e8b8b0dfe13c68ce5b6]
4.19-upstream-stable: released (4.19.193) [0f2cb08c57edefb0e7b5045e0e3e9980a3d3aa37]
sid: released (5.14.6-1)
6.1-bookworm-security: N/A "Fixed before branching point"
5.10-bullseye-security: released (5.10.46-1)
4.19-buster-security: released (4.19.194-1)