os/FileStore: reset journal state on umount

We observed a sequence like:

 - replay journal
   - sets JournalingObjectStore applied_op_seq
 - umount
 - mount
   - initiate commit with prevous applied_op_seq
 - replay journal
   - commit finishes
   - on replay commit, we fail assert op > committed_seq

Although strictly speaking the assert failure is harmless here, in general
we should not let state leak through from a previous mount into this
mount or else assertions are in general more difficult to reason about.

Fixes: #8019
Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 4de49e8676748b6ab4716ff24fd0a465548594fc)