os/FileStore: fix mount/remount force_sync race

Consider:

 - mount
 - sync_entry is doing some work
 - umount
   - set force_sync = true
   - set done = true
 - sync_entry exits (due to done)
   - ..but does not set force_sync = false
 - mount
 - journal replay starts
 - sync_entry sees force_sync and does a commit while op_seq == 0
 ...crash...

Fixes: #9144
Backport: firefly, dumpling
Signed-off-by: Sage Weil <sage@redhat.com>
os/FileStore: rename start_sync() -> do_force_sync()

Just to avoid confusion with start_sync(Context*c), which is totally
different.

Signed-off-by: Sage Weil <sage@redhat.com>
Merge pull request #2274 from ceph/wip-9144

filestore: fix force_sync race with mount + umount + mount

Reviewed-by: David Zafman <dzafman@redhat.com>