[XFS] Don't use kmap in xfs_iozero.

kmap() is inefficient and does not scale well. kmap_atomic() is a better
choice. Use the generic wrapper function instead of open coding the
kmap-memset-dcache flush-kunmap stuff.

SGI-PV: 960904
SGI-Modid: xfs-linux-melb:xfs-kern:28041a

Signed-off-by: David Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>