Stephen Tweedie reports a 2.4.7 problem in which kswapd is chewing lots
of CPU trying to reclaim inodes which are pinned by buffer_heads at
i_dirty_buffers.

This can only happen when there's memory pressure on ZONE_HIGHMEM - the
2.4 kernel runs shrink_icache_memory in that case as well.  But there's
no reclaim pressure on ZONE_NORMAL so the VM is never running
try_to_free_buffers() against the ZONE_NORMAL buffers which are pinning
the inodes.

The 2.5 kernel also runs the slab shrinkers in response to ZONE_HIGHMEM
pressure.  This may be wrong - still thinking about that.

This patch arranges for prune_icache to try to remove the inode's buffers
when the inode is to be reclaimed.

It also changes inode_has_buffers() and the other inode-buffer-list
functions to look at inode->i_data, not inode->i_mapping.  The latter
was wrong.