Fix -Wattribute-warning in edonr

The wrong union memory was being accessed in EdonRInit resulting in
a write beyond size of field compiler warning. Reference the correct
member to resolve the warning. The warning was correct and this in
case the mistake was harmless.

In function ‘fortify_memcpy_chk’,
inlined from ‘EdonRInit’ at zfs/module/icp/algs/edonr/edonr.c:494:3:
./include/linux/fortify-string.h:344:25: error: call to
‘__write_overflow_field’ declared with attribute warning:
detected write beyond size of field (1st parameter);
maybe use struct_group()? [-Werror=attribute-warning]

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #13528
Closes #13575