clang does not like flexible array in the struct

[youzhongyang]

Signed-off-by: Youzhong Yang yyang@mathworks.com

Motivation and Context

#14737 breaks FreeBSD build when clang is used, we need to make both Linux and FreeBSD happy.

Description

Add flexible array union member in struct dnode_phys_t for gcc only.

How Has This Been Tested?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[lundman]

There is also -Wno-gnu-variable-sized-type-not-at-end in that, we know they are flexible but we aren't to use them that way, in this struct :) (perhaps with a #pragma clang diagnostic ignored "-Wno-gnu-variable-sized-type-not-at-end")
Similar to how we handle zfs_notreached or noreturn ?

[behlendorf]

@lundman's suggestion seems preferable to me since it'd be nice to keep this code consistent across platforms.

[behlendorf]

Closing in favor of #14764 .