xfsprogs-6.10 breaks C++ compilation

[Sam James <sam@gentoo.org>]

Hi!

Forwarding a downstream report from https://bugs.gentoo.org/938569.

xfs_fs.h's xfs_getparents_next_rec relies on an implicit void*-to-T*
cast which is legal in C, but not C++.

It fails like so when a C++ project tries to use the header:
```
/usr/include/xfs/xfs_fs.h: In function 'xfs_getparents_rec* xfs_getparents_next_rec(xfs_getparents*, xfs_getparents_rec*)':
/usr/include/xfs/xfs_fs.h:915:16: error: invalid conversion from 'void*' to 'xfs_getparents_rec*' [-fpermissive]
  915 |         return next;
      |                ^~~~
      |                |
      |                void*

```

It came up when building libktorrent.

thanks,
sam