Implements a new set of block address_space_operations which will never
attach buffer_heads to file pagecache.  These can be turned on for ext2
with the `nobh' mount option.

During write-intensive testing on a 7G machine, total buffer_head
storage remained below 0.3 megabytes.  And those buffer_heads are
against ZONE_NORMAL pagecache and will be reclaimed by ZONE_NORMAL
memory pressure.

This work is, of course, a special for the huge highmem machines.
Possibly it obsoletes the buffer_heads_over_limit stuff (which doesn't
work terribly well), but that code is simple, and will provide relief
for other filesystems.


It should be noted that the nobh_prepare_write() function and the
PageMappedToDisk() infrastructure is what is needed to solve the
problem of user data corruption when the filesystem which backs a
sparse MAP_SHARED mapping runs out of space.  We can use this code in
filemap_nopage() to ensure that all mapped pages have space allocated
on-disk.  Deliver SIGBUS on ENOSPC.

This will require a new address_space op, I expect.