zdtm: mountpoints -- Explicitly remove old /dev/null

We should explicitly unlink old /dev/null instance, otherwise
if such device already exist we might hit a problem as

 | (00.002984) 7412 fdinfo 0: pos: 0x               0 flags:           100002/0
 | (00.003015) Dumping path for 0 fd via self 42 [/dev/null]
 | (00.003025) Error (files-reg.c:422): Unaccessible path opened 2049:109720, need 5:10
 | (00.003033) Error (cr-dump.c:1565): Dump files (pid: 7412) failed with -1

Reported-by: Alexander Kartashov <alekskartashov@parallels.com>
Tested-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>