ipv4: initialize flowi4_flags before calling fib_lookup()

ipv4: initialize flowi4_flags before calling fib_lookup()

Field fl4.flowi4_flags is not initialized in fib_compute_spec_dst()
before calling fib_lookup(), which means fib_table_lookup() is
using non-deterministic data at this line:

	if (!(flp->flowi4_flags & FLOWI_FLAG_SKIP_NH_OIF)) {

Fix by initializing the entire fl4 structure, which will prevent
similar issues as fields are added in the future by ensuring that
all fields are initialized to zero unless explicitly initialized
to another value.

Fixes: c8e929e639321 ("net: Fix vti use case with oif in dst lookups")
Suggested-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: Lance Richardson <lrichard@redhat.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>