Linux 6.3 compat: Fix memcpy "detected field-spanning write" error

Add a new union member of flexible array to dnode_phys_t and use
it in the macro so we can silence the memcpy() fortify error.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Youzhong Yang <yyang@mathworks.com>
Closes #14737