ploop attach: fix handling short buffer
ploop attach: fix handling short buffer

If the buffer provided is too short, we shouldn't use it but return an
error.

Also, improve a comment explaining error checking sequence.

Signed-off-by: Kir Kolyshkin <c0159612b0ef32139322233232df1a0d189c4b9b@openvz.org>

xemul/mosaic,xemul/mosaic,kolyshkin/mosaic,kolyshkin/mosaic,xemul/mosaic,kolyshkin/mosaic