block/nfs: limit maximum readahead size to 1MB

block/nfs: limit maximum readahead size to 1MB

a malicious caller could otherwise specify a very
large value via the URI and force libnfs to allocate
a large amount of memory for the readahead buffer.

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Lieven <pl@kamp.de>
Message-id: 1435317241-25585-1-git-send-email-pl@kamp.de
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
(cherry picked from commit 29c838cdc96c4d117f00c75bbcb941e1be9590fb)
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>