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

drivers/staging/exfat/exfat_super.c

@@ -743,8 +743,10 @@ static int ffsReadFile(struct inode *inode, struct file_id_t *fid, void *buffer,
 
 			while (clu_offset > 0) {
 				/* clu = FAT_read(sb, clu); */
-				if (FAT_read(sb, clu, &clu) == -1)
-					return FFS_MEDIAERR;
+				if (FAT_read(sb, clu, &clu) == -1) {
+					ret = FFS_MEDIAERR;
+					goto out;
+				}
 
 				clu_offset--;
 			}