ztest: comparisons against errno should not assign to it

888914486e26d81c8dbfd7a9d8091c05f9fc98ba introduced this regression.

I used cscope to verify that there are no other instances of this in the
codebase. This is the one of the few bugs that are extremely easy to
identify using cscope.

Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@alumni.stonybrook.edu>
Closes #14264