client: stop the remount_finisher thread in the Client::unmount()

client: stop the remount_finisher thread in the Client::unmount()

The ceph_fuse will unmount the client and then finalize the cfuse
and at the same will free the mountpoint memory. And at last will
try to stop the remount_finisher thread. But the remount_finisher
thread will use the freed mountpoint to do the remount, which will
case unexpected remount failures.

Just stop the remount_finisher thread in the Client::unmount().

Fixes: https://tracker.ceph.com/issues/56249
Signed-off-by: Xiubo Li <xiubli@redhat.com>