Disable sparse_001_pos / truncate_001_pos.ksh on 32-bit systems

Commit 539d33c seems to have significantly increased the run time
of the sparse_001_pos.ksh and truncate_001_pos.ksh test cases on
32-bit systems. This is likely due to dirty blocks from frees
being deferred to later txgs.

At the moment this is resulting in frequent failures on the
32-bit builders. Disable this test case until the issue can be
analyzed and resolved.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #5727
Closes #5728