os/FileJournal: limit size of aio submission

Limit size of each aio submission to IOV_MAX-1 (to be safe).  Take care to
only mark the last aio with the seq to signal completion.

Signed-off-by: Sage Weil <sage@inktank.com>
test_filejournal: test journaling bl with >IOV_MAX segments

Signed-off-by: Sage Weil <sage@inktank.com>
test_filejournal: optionally specify journal filename as an argument

Signed-off-by: Sage Weil <sage@inktank.com>
Merge branch 'wip-journal-aio' into next

Reviewed-by: Samuel Just <sam.just@inktank.com>