UPSTREAM: net: l3mdev: Add master device lookup by index

Add helper to lookup l3mdev master index given a device index.

[cherry-pick of upstream 1a8524794fc7c70f44ac28e3a6e8fd637bc41f14]

Bug: 63589535
Change-Id: I3d0758a5d0eb03791726014c9c1e32e187391e6f
Signed-off-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>