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>
(cherry picked from commit dd11042)

Conflicts:
	src/os/FileStore.cc

src/os/FileStore.cc

@@ -1558,6 +1558,8 @@ int FileStore::umount()
     backend = generic_backend;
   }
 
+  force_sync = false;
+
   object_map.reset();
 
   {