grow_buffers() infinite loop fix (CVE-2006-5757/CVE-2006-6060)

If grow_buffers() is for some reason passed a block number which wants to li
outside the maximum-addressable pagecache range (PAGE_SIZE * 4G bytes) then
will accidentally truncate `index' and will then instnatiate a page at the
wrong pagecache offset.  This causes __getblk_slow() to go into an infinite
loop.

This can happen with corrupted disks, or with software errors elsewhere.

Detect that, and handle it.

Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>