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).

The call to align_bl() is moved into do_write in order to ensure that write_pos
is adjusted correctly prior to write_bl().

Also, we adjust pos at the end of write_bl() such that pos \in [get_top(),
header.max_size) after write_bl().

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