xfs: kill VN_DIRTY()

Only one user of the macro and the dirty mapping check is redundant
so just get rid of it.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <david@fromorbit.com>

diff --cc fs/xfs/xfs_inode.c
Simple merge

diff --cc fs/xfs/xfs_vnode.h
Simple merge