[XFS] Kill SYNC_CLOSE

SYNC_CLOSE is only ever used and checked in conjunction with SYNC_WAIT,
and this only done in one spot. The only thing this does is make
XFS_bflush() calls to the data buftargs.

This will happen very shortly afterwards the xfs_sync() call anyway in the
unmount path via the xfs_close_devices(), so this code is redundant and
can be removed. That only user of SYNC_CLOSE is now gone, so kill the flag
completely.

SGI-PV: 988140

SGI-Modid: xfs-linux-melb:xfs-kern:32310a

Signed-off-by: David Chinner <david@fromorbit.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>