With preallocation turned on, reiserfs_allocate_blocks_for_region wasn't
sending a large enough array to hold all the blocks it was asking the block
allocator to find.  This can result in stack corruption.

The fix is to kmalloc an array to hold the blocks, making sure to allocate
something large enough.

There was also a recent optimization to force the allocator to find a free
region large enough to hold the entire preallocation size.  This was
sometimes causing more blocks to be allocated then had been requested,
which would also overflow the array.  Something more elegant is required
here, until then just disable the optimization.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>