selftests: bpf: test_sockmap: handle file creation failures gracefully

test_sockmap creates a temporary file to use for sendpage.
this may fail for various reasons. Handle the error rather
than segfault.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>