exfat: don't RCU-free the sbi

There are no RCU critical sections for accessing any information in the
sbi, so drop the call_rcu indirection for freeing the sbi.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Message-Id: <20230809220545.1308228-10-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>