Fix -Wattribute-warning in dsl layer

The memcpy(), memmove(), and memset() functions have been annotated
to perform bounds checking when using FORTIFY_SOURCE. A warning is
now generted when writing beyond the end of the specified field.

Alternately, the new struct_group() macro could be used to create
an anonymous union member for use by memcpy(). However, since this
is the only place the macro would be helpful it's preferable to
restructure the code slights to avoid the need for additional
compatibility code when the macro does not exist.

https://lore.kernel.org/lkml/20211118183807.1283332-1-keescook@chromium.org/T/

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