Using <linux/stat.h> for _GNU_SOURCE

[Florian Weimer]

Would it be possible to include <linux/stat.h> (not <asm/stat.h>) from 
<sys/stat.h> for _GNU_SOURCE?  We'd need to add redefinition guards for 
the S_* macros defined in <linux/stat.h>, but otherwise, the header 
looks quite clean.

This would allow us to obtain the STX_* constants and struct statx from 
the uapi header, without maintaining our own copy of that information. 
Or wouldn't be the __u* types in struct statx not acceptable to us?

Thanks,
Florian