Patch from Badari Pulavarty <pbadari@us.ibm.com> and myself.

This patch adds AIO support for direct-IO - O_DIRECT files, O_DIRECT
blockdevs and /dev/raw/rawN.

Also, the controlling `struct dio' is now always kmalloced, saving ~400 bytes
of stack.

The best tool for testing AIO/DIO is the modified fsx-linux in ext3 CVS.
See http://www.zip.com.au/~akpm/linux/ext3/ for details.  For example:

	fsx-linux -Z -A -R -W -r 512 -w 4096 foo

Running many instances of this against ext3 currently fails with incorrect
file data; some bug in the new ext3 O_DIRECT support.  ext2 is OK though.