grace: replace BUG_ON by WARN_ONCE in exit_net hook

[ Upstream commit b872285751c1af010e12d02bce7069e2061a58ca ]

Signed-off-by: Vasily Averin <vvs@virtuozzo.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>