unix_diag: fix incorrect sign extension in unix_lookup_by_ino

The value passed by unix_diag_get_exact to unix_lookup_by_ino has type
__u32, but unix_lookup_by_ino's argument ino has type int, which is not
a problem yet.
However, when ino is compared with sock_i_ino return value of type
unsigned long, ino is sign extended to signed long, and this results
to incorrect comparison on 64-bit architectures for inode numbers
greater than INT_MAX.

This bug was found by strace test suite.

Fixes: 5d3cae8b ("unix_diag: Dumping exact socket core")
Signed-off-by: Dmitry V. Levin <ldv@altlinux.org>
Acked-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>