[XFS] wire up ->open for directories

Currently there's no ->open method set for directories on XFS.  That
means we don't perform any check for opening too large directories
without O_LARGEFILE, we don't check for shut down filesystems, and we
don't actually do the readahead for the first block in the directory.

Instead of just setting the directories open routine to xfs_file_open
we merge the shutdown check directly into xfs_file_open and create
a new xfs_dir_open that first calls xfs_file_open and then performs
the readahead for block 0.

(First sent on September 29th)

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <david@fromorbit.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>