os/FileStore: fix non-btrfs op_seq commit order

The op_seq file is the starting point for journal replay.  For stable btrfs
commit mode, which is using a snapshot as a reference, we should write this
file before we take the snap.  We normally ignore current/ contents anyway.

On non-btrfs file systems, however, we should only write this file *after*
we do a full sync, and we should then fsync(2) it before we continue
(and potentially trim anything from the journal).

This fixes a serious bug that could cause data loss and corruption after
a power loss event.  For a 'kill -9' or crash, however, there was little
risk, since the writes were still captured by the host's cache.

Fixes: #3721
Signed-off-by: Sage Weil <sage@inktank.com>