staging: exfat: Ensure we unlock upon error in ffsReadFile

The call was not releasing the mutex upon error.

Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Acked-By: Valdis Kletnieks <valdis.kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191103180921.2844-1-dave@stgolabs.net


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