UBUNTU: SAUCE: Fix non-prefaulted page deadlock (LP: #1754584)

UBUNTU: SAUCE: Fix non-prefaulted page deadlock (LP: #1754584)

BugLink: https://bugs.launchpad.net/bugs/1754584
Fix mmap'd libaio read on non-prefaulted page deadlock. This is a hot fix
from ZFS upstream that ensure pages do not deadlock and replaces the original
fix as it is a far better solution.

Performing a read with the target data in a mmap'd page that is map'd into
the same blocks that are being read causes a lock on the page and a further
lock on the same page when the page is being faulted in, causing deadlock.

This is an improved fix from the original fix which ensures the pages don't
deadlock and removes the original potentially racy pre-faulting fix

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>