This patch addresses the excessive consumption of ZONE_NORMAL by
buffer_heads on highmem machines.  The algorithms which decide which
buffers to shoot down are fairly dumb, but they only cut in on machines
with large highmem:lowmem ratios and the code footprint is tiny.

The buffer.c change implements the buffer_head accounting - it sets the
upper limit on buffer_head memory occupancy to 10% of ZONE_NORMAL.

A possible side-effect of this change is that the kernel will perform
more calls to get_block() to map pages to disk.  This will only be
observed when a file is being repeatadly overwritten - this is the only
case in which the "cached get_block result" in the buffers is useful.

I did quite some testing of this back in the delalloc ext2 days, and
was not able to come up with a test in which the cached get_block
result was measurably useful.  That's for ext2, which has a fast
get_block().

A desirable side effect of this patch is that the kernel will be able
to cache much more blockdev pagecache in ZONE_NORMAL, so there are more
ext2/3 indirect blocks in cache, so with some workloads, less I/O will
be performed.

In mpage_writepage(): if the number of buffer_heads is excessive then
buffers are stripped from pages as they are submitted for writeback.
This change is only useful for filesystems which are using the mpage
code.  That's ext2 and ext3-writeback and JFS.  An mpage patch for
reiserfs was floating about but seems to have got lost.

There is no need to strip buffers for reads because the mpage code does
not attach buffers for reads.

These are perhaps not the most appropriate buffer_heads to toss away.
Perhaps something smarter should be done to detect file overwriting, or
to toss the 'oldest' buffer_heads first.

In refill_inactive(): if the number of buffer_heads is excessive then
strip buffers from pages as they move onto the inactive list.  This
change is useful for all filesystems.  This approach is good because
pages which are being repeatedly overwritten will remain on the active
list and will retain their buffers, whereas pages which are not being
overwritten will be stripped.