xfs: add a define for statfs magic to uapi

Needed by userspace programs that call fstatfs().

It'd be natural to publish XFS_SB_MAGIC in uapi, but while these two
have identical values, they have different semantic meaning: one is
an enum cookie meant for statfs, the other a signature of the
on-disk format.

Signed-off-by: Adam Borowski <kilobyte@angband.pl>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>