Now that kernel exports linux/mount.h and includes it on linux/fs.h,
its definitions might clash with glibc exports sys/mount.h. To avoid
the need to rearrange the Linux header to be always after glibc one,
the glibc sys/mount.h is changed to:
1. Undefine the macros also used as enum constants. This covers prior
inclusion of <linux/mount.h> (for instance MS_RDONLY).
2. Include <linux/mount.h> based on the usual __has_include check
(needs to use __has_include ("linux/mount.h") to paper over GCC
bugs.
3. Define enum fsconfig_command only if FSOPEN_CLOEXEC is not defined.
(FSOPEN_CLOEXEC should be a very close proxy.)
4. Define struct mount_attr if MOUNT_ATTR_SIZE_VER0 is not defined.
(Added in the same commit on the Linux side.)
This patch also adds some tests to check if including linux/fs.h and
linux/mount.h after and before sys/mount.h does work.
Checked on x86_64-linux-gnu.
Reviewed-by: Florian Weimer <fweimer@redhat.com>
