ip: lwtunnel: seg6: bail out if table ids are invalid

ip: lwtunnel: seg6: bail out if table ids are invalid

When table and vrftable are used in SRv6, ip should bail out if table
ids are not valid, and return a proper error message to the user.

Achieve this simply checking rtnl_rttable_a2n return value, as we
already do in the rest of iproute.

Fixes: 0486388a877a ("add support for table name in SRv6 End.DT* behaviors")
Fixes: 69629b4e43c4 ("seg6: add support for vrftable attribute in SRv6 End.DT4/DT6 behaviors")
Signed-off-by: Andrea Claudi <aclaudi@redhat.com>
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>