gre: Prepare ipgre_open() to .flowi4_tos conversion.

gre: Prepare ipgre_open() to .flowi4_tos conversion.

Use ip4h_dscp() to get the tunnel DSCP option as dscp_t, instead of
manually masking the raw tos field with INET_DSCP_MASK. This will ease
the conversion of fl4->flowi4_tos to dscp_t, which just becomes a
matter of dropping the inet_dscp_to_dsfield() call.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/6c05a11afdc61530f1a4505147e0909ad51feb15.1736941806.git.gnault@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>