exfat: free the sbi and iocharset in ->kill_sb

As a rule of thumb everything allocated to the fs_context and moved into
the super_block should be freed by ->kill_sb so that the teardown
handling doesn't need to be duplicated between the fill_super error
path and put_super.  Implement an exfat-specific kill_sb method to do
that and share the code with the mount contex free helper for the
mount error handling case.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Message-Id: <20230809220545.1308228-11-hch@lst.de>
Signed-off-by: Christian Brauner <brauner@kernel.org>