FileJournal: adjust write_pos prior to unlocking write_lock

In committed_thru, we use write_pos to reset the header.start
value in cases where seq is past the end of our journalq.
It is therefore important that the journalq be updated
atomically with write_pos (that is, under the write_lock).

Fixes: #5020
Signed-off-by: Samuel Just <sam.just@inktank.com>