Merge branch 'ipv4-convert-__fib_validate_source-and-its-callers-to-dscp_t'

Guillaume Nault says:

====================
ipv4: Convert __fib_validate_source() and its callers to dscp_t.

This patch series continues to prepare users of ->flowi4_tos to a
future conversion of this field (__u8 to dscp_t). This time, we convert
__fib_validate_source() and its call chain.

The objective is to eventually make all users of ->flowi4_tos use a
dscp_t value. Making ->flowi4_tos a dscp_t field will help avoiding
regressions where ECN bits are erroneously interpreted as DSCP bits.
====================

Link: https://patch.msgid.link/cover.1728302212.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>