sys/sysinfo.h clash with linux/kernel.h

[Rich Felker]

linux/kernel.h is a uapi header that does almost nothing but define
some internal-use alignment macros and -- oddly -- include
linux/sysinfo.h to provide a definition of struct sysinfo. It's
included only from 6 places in the kernel uapi headers:

include/uapi/linux/lightnvm.h
include/uapi/linux/ethtool.h
include/uapi/linux/sysctl.h
include/uapi/linux/netlink.h
include/uapi/linux/netfilter/x_tables.h
include/uapi/linux/mroute6.h

However, it's also included from glibc's sys/sysinfo.h to provide
struct sysinfo (glibc depends on the kernel for the definition). On
musl, this produces a conflicting definition if both sys/sysinfo.h and
any of the above 6 headers are included in the same file.

I think the underlying problem here is that the same header is used
for two very disjoint purposes: by glibc as the provider of struct
sysinfo, and by other kernel headers as provider of the alignment
macros.

The glibc use is effectively a permanent contract that can't be
changed, so what I'd like to do is move the macros out to a separate
header (maybe linux/something_macros.h?) and have linux/kernel.h and
the above 6 uapi headers all include that. Then nothing but
linux/kernel.h would pull in linux/sysinfo.h.

Note that in practice this is a rather hard issue to hit since almost
nothing needs sysinfo() at the same time as the above uapi interfaces.
However it did come up in toybox, which is how I first (just today)
learned about the conflict, so it seems like something that should be
fixed.

Rich