xfs: fix C++ compilation errors in xfs_fs.h

xfs: fix C++ compilation errors in xfs_fs.h

Several people reported C++ compilation errors due to things that C
compilers allow but C++ compilers do not.  Fix both of these problems,
and hope there aren't more of these brown paper bags in 2 months when we
finally get these fixes through the process into a released xfsprogs.

NOTE: I am submitting this bugfix over the objections of a former
maintainer, who insists that we should remove this function from the
published userspace ABI instead of fixing the C++ compilation errors.
No deprecation period, no discussion, just a hard drop of an already
provided and correct C function, which would be in contravention of
Linus' rules.  IOWs, removing ABI that have already shipped in a
released kernel requires a careful deprecation period, so I will let
that maintainer run that process.

Reported-by: kernel@mattwhitlock.name
Reported-by: sam@gentoo.org
Closes: https://bugzilla.kernel.org/show_bug.cgi?id=219203
Fixes: 233f4e12bbb2c ("xfs: add parent pointer ioctls")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>