staging: exfat: Clean up return codes - FFS_MEDIAERR

Convert FFS_MEDIAERR to (mostly) -ENOENT and -EIO.  Some additional code surgery
needed to propogate correct error codes upwards.

Signed-off-by: Valdis Kletnieks <Valdis.Kletnieks@vt.edu>
Link: https://lore.kernel.org/r/20191112021000.42091-3-Valdis.Kletnieks@vt.edu


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