lslocks: don't abort gathering per-process information even if opening a /proc/[0...

lslocks: don't abort gathering per-process information even if opening a /proc/[0-9]* fails

If a process ($pid) taking a lock is gone while running lslocks,
gathering per-process information for the process may fail in opening
/proc/$pid. Though lslocks should work with incomplete information,
the original code stopped gathering per-process information for the
other processes.

As a result of the original behavior, tests/ts/lslocks/lslocks really
failed in an environment where multiple test cases ran simultaneously.

Close #2624 again.
See also #2633.

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