os/FileJournal: logger is optional

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 076b418c7f03c5c62f811fdc566e4e2b776389b7)
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>
(cherry picked from commit dda7b651895ab392db08e98bf621768fd77540f0)
test_filejournal: test journaling bl with >IOV_MAX segments

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit c461e7fc1e34fdddd8ff8833693d067451df906b)
test_filejournal: optionally specify journal filename as an argument

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 483c6f76adf960017614a8641c4dcdbd7902ce33)