ocfs2: add support for read_iter, write_iter, and direct_IO_bvec

ocfs2: add support for read_iter, write_iter, and direct_IO_bvec

ocfs2's .aio_read and .aio_write methods are changed to take
iov_iter and pass it to generic functions.  Wrappers are made to pack
the iovecs into iters and call these new functions.

ocfs2_direct_IO() is trivial enough that a new function is made which
passes the bvec down to the generic direct path.

Signed-off-by: Zach Brown <zach.brown@oracle.com>
Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>