t_attr_corruption: fix this yet again

t_attr_corruption: fix this yet again

Jeff Mahoney pointed out that 'security.evm' actually has an expected
value format, which breaks the test if EVM is enabled.  It turns out
that the 'security.evm' setxattr call in the original syzkaller report
was a total red herring, as this bug can be reproduced without it.

Fix the test case to do the minimum amount of work needed to reproduce
the corruption.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Allison Henderson <allison.henderson@oracle.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>