[PATCH 5.10 0/1] net/mlx5e: Fix nullptr in mlx5e_tc_add_fdb_flow()

[Nikita Zhandarovich]

The result of __dev_get_by_index() is not checked for NULL, which then
passed to mlx5e_attach_encap() and gets dereferenced.

This patch backports commit <fe7738e> to correct the issue in 5.10 branch.

Found by Linux Verification Center (linuxtesting.org) with SVACE.