netlink: Don't use a zero-length array

[jhb]

Summary

Define SNL_DECLARE_FIELD_PARSER* macros to create a parser that has
no output attributes only input fields and use this to define the
snl_donemsg_parser.

This removes the need for the zero-length nla_p_donemsg[] variable.
Zero length arrays are not valid in ISO C.

[jrtc27]

sys/netlink/netlink_snl.h

201

I can't obviously see a reason why out_size wouldn't be useful for a field-only parser. Both snl_field_parser and snl_attr_parser have callbacks that take the target allocated based on out_size.

Also field generally becomes before attr in the file AFAICT, so would suggest moving this code to line 176.

[jhb]

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[jhb]

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