scripts: Fix regression in VLAN interface support

scripts: Fix regression in VLAN interface support

Commit 6471541d6d2bc9f2af0ff92b280abbd1d933cf88 broke support for VLAN
interfaces.  Releasing a public IP address depends on
ip_maskbits_iface() and for a VLAN interface this will return an
interface of the form <vlan>@<iface>, which can't be fed back into
"ip" commands.

Update ip_maskbits_iface() to drop the '@' and everything after it.

Signed-off-by: Martin Schwenke <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
Reported-by: Jan Schwaratzki <jschwaratzki@ddn.com>
(Imported from commit 87c5c96b767aa317dd620f89ac3e11bb40dae70f)