prepare_pstree: fixup reading kernel pid_max

Two fixes (reported by coverity) and a minor nitpick:

1. Fix checking error from open_proc().

2. Fix buffer overflow. MAX_ULONG can be 20 characters long, so
ret = read() can return 20 and buf[ret] = 0 will overrun the buf.
Make a buf one character longer (an extra byte for \0) and pass
sizeof(buf) - 1 to read to fix it.

3. Call close() right after read().

This is a fixup to commit e68bded.

Reported by Coverity, CID 168505, 168504.

Cc: Laurent Dufour <ldufour@linux.vnet.ibm.com>
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>