staging: exfat: Fix two missing unlocks on error paths

These two error paths need to unlock before we can return.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20190904095908.GA7007@mwanda


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>