vxlan: Insert FDB into hash table in vxlan_fdb_create()

vxlan: Insert FDB into hash table in vxlan_fdb_create()

Commit 7c31e54aeee5 ("vxlan: do not destroy fdb if register_netdevice()
is failed") split the insertion of FDB entries into the FDB hash table
from the function where they are created.

This was done in order to work around a problem that is no longer
possible after the previous patch. Simplify the code and move the body
of vxlan_fdb_insert() back into vxlan_fdb_create().

Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20250415121143.345227-4-idosch@nvidia.com
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>