sctp: Prepare sctp_v4_get_dst() to dscp_t conversion.

sctp: Prepare sctp_v4_get_dst() to dscp_t conversion.

Define inet_sk_dscp() to get a dscp_t value from struct inet_sock, so
that sctp_v4_get_dst() can easily set ->flowi4_tos from a dscp_t
variable. For the SCTP_DSCP_SET_MASK case, we can just use
inet_dsfield_to_dscp() to get a dscp_t value.

Then, when converting ->flowi4_tos from __u8 to dscp_t, we'll just have
to drop the inet_dscp_to_dsfield() conversion function.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/1a645f4a0bc60ad18e7c0916642883ce8a43c013.1735835456.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>