xfs: move I/O type flags into xfs_aops.c

The IOMAP_ flags are now only used inside xfs_aops.c for extent
probing and I/O completion tracking, so more them here, and rename
them to IO_* as there's no mapping involved at all.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Alex Elder <aelder@sgi.com>