zdb_il: use flex array member to access ZIL records

In 6f50f8e16 we added flex arrays to lr_XX_t structs to silence kernel
bounds check warnings. Userspace code was mostly not updated to use them
though.

It seems that in the right circumstances, compilers can get confused
about sizes in the same way, and throw warnings. This commits switch
those uses over to use the flex array fields also.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16832