generic/{436,445}: define _cleanup function earlier in the tests

generic/{436,445}: define _cleanup function earlier in the tests

If the filesystem doesn't support a feature that is required for the tests
to run, they will fail to execute the _cleanup function because it isn't yet
defined:

  ./common/rc: line 1: _cleanup: command not found

This error became more visible with commit 87a53d2e7c82 ("generic/{436,445}:
check falloc support").

Cc: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>