osdc: fix Journaler write error handling

Since we started wrapping the write error
handler in a finisher, multiple calls to
handle_write_error would hit the assert()
on the second call before the actual
handler had been called (at the other end
of the finisher) from the first call.

The symptom was that the MDS was intermittently
failing to respawn on blacklist, seen in #10011.

Signed-off-by: John Spray <john.spray@redhat.com>