IB/addr: Fix setting source address in addr6_resolve()

Commit eea40b8f ("infiniband: call ipv6 route lookup via the stub
interface") introduced a regression in address resolution when connecting
to IPv6 destination addresses.  The old code called ip6_route_output(),
while the new code calls ipv6_stub->ipv6_dst_lookup().  The two are almost
the same, except that ipv6_dst_lookup() also calls ip6_route_get_saddr()
if the source address is in6addr_any.

This means that the test of ipv6_addr_any(&fl6.saddr) now never succeeds,
and so we never copy the source address out.  This ends up causing
rdma_resolve_addr() to fail, because without a resolved source address,
cma_acquire_dev() will fail to find an RDMA device to use.  For me, this
causes connecting to an NVMe over Fabrics target via RoCE / IPv6 to fail.

Fix this by copying out fl6.saddr if ipv6_addr_any() is true for the original
source address passed into addr6_resolve().  We can drop our call to
ipv6_dev_get_saddr() because ipv6_dst_lookup() already does that work.

Fixes: eea40b8f ("infiniband: call ipv6 route lookup via the stub interface")
Cc: <stable@vger.kernel.org> # 3.12+
Signed-off-by: Roland Dreier <roland@purestorage.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>