xfs: turn XFS_ATTR3_RMT_BUF_SPACE into a function

Turn this into a properly typechecked function, and actually use the
correct blocksize for extended attributes.  The function cannot be
static inline because xfsprogs userspace uses it.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Andrey Albershteyn <aalbersh@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>