xfs: remove unused values from xfs_exntst_t

We only ever use the normal and unwritten states.  And the actual
ondisk format (this enum isn't despite being in xfs_format.h) only
has space for the unwritten bit anyway.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>