zdtm: Make file_locks04 test based on /proc/{pid}/fdinfo/{fd}

/proc/locks is racy with adding/removing locks,
so we may lose lock on check.

Use fdinfo's list of locks instead.

v2: Feature 'fdinfo_lock'

Signed-off-by: Kirill Tkhai <ktkhai@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>