generic/504: fix hard coded fd number

Bash supports file discriptor assignment in this way. So remove the hard
coded numbers. Also close this opened fd in cleanup.

Signed-off-by: Murphy Zhou <xzhou@redhat.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>

tests/generic/504

@@ -21,6 +21,7 @@ trap "_cleanup; exit \$status" 0 1 2 3 15
 
 _cleanup()
 {
+	exec {test_fd}<&-
 	cd /
 	rm -f $tmp.*
 }
@@ -45,9 +46,9 @@ tf_inode=$(stat -c %i $testfile)
 echo inode $tf_inode >> $seqres.full
 
 # Create new fd by exec
-exec 9> $testfile
+exec {test_fd}> $testfile
 # flock locks the fd then exits, we should see the lock info even the owner is dead
-flock -x 9
+flock -x $test_fd
 cat /proc/locks >> $seqres.full
 
 # Checking