lsns: continue the executing even if opening a /proc/$pid fails

lsns: continue the executing even if opening a /proc/$pid fails

In the original code, lsns printed nothing if it failed in opening the
last dntry in /proc/[0-9]* though lsns should work partially.

The original behavior caused the combination of the following two
test cases failed:

    $ tests/ts/lsns/filter & tests/ts/lsns/ioctl_ns &
    [1] 19178
    [2] 19179
    $          lsns: ownership and hierarchy        ...         \
    lsns: -Q, --filter option            ... FAILED
     FAILED

    [1]-  Done                    tests/ts/lsns/filter
    [2]+  Done                    tests/ts/lsns/ioctl_ns

Signed-off-by: Masatake YAMATO <yamato@redhat.com>