Workaround UBSAN errors for variable arrays

This gets around UBSAN errors when using arrays at the end of
structs. It converts some zero-length arrays to variable length
arrays and disables UBSAN checking on certain modules.

It is based off of the patch from #15460.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Tested-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Co-authored-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Issue #15145
Closes #15510