[NET]: Fix dentry leak in sys_accept().

This regression was added by commit:
39d8c1b6
("Do not lose accepted socket when -ENFILE/-EMFILE.")

This is based upon a patch from Andi Kleen.

Thanks to Adrian Bridgett for narrowing down a good test case, and to
Andi Kleen and Andrew Morton for eyeballing this code.

Signed-off-by: David S. Miller <davem@davemloft.net>