Fix atomic_clear_long_excl wrong return

When clearing a bit, we should check whether that bit is 0.
Note atomic_clear_long_excl is not used.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Liu Qing <winglq@gmail.com>
Closes #10526