selftests: use posix-style redirection in ip_defrag.sh

The ip_defrag.sh script requires bash-style output redirection but
use the default shell. This may cause random failures if the default
shell is not bash.
Address the above using posix compliant output redirection.

Fixes: 02c7f38b ("selftests/net: add ip_defrag selftest")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>