block: add some comments around the bio read-write flags

/*

 * bio bi_rw flags

 *

 * bit 0 -- read (not set) or write (set)

 * bit 0 -- data direction

 *	If not set, bio is a read from device. If set, it's a write to device.

 * bit 1 -- rw-ahead when set

 * bit 2 -- barrier

 *	Insert a serialization point in the IO queue, forcing previously

 *	submitted IO to be completed before this oen is issued.

 * bit 3 -- fail fast, don't want low level driver retries

 * bit 4 -- synchronous I/O hint: the block layer will unplug immediately

 *	Note that this does NOT indicate that the IO itself is sync, just

 *	that the block layer will not postpone issue of this IO by plugging.

 * bit 5 -- metadata request

 *	Used for tracing to differentiate metadata and data IO. May also

 *	get some preferential treatment in the IO scheduler

 * bit 6 -- discard sectors

 *	Informs the lower level device that this range of sectors is no longer

 *	used by the file system and may thus be freed by the device. Used

 *	for flash based storage.

 */

#define BIO_RW		0	/* Must match RW in req flags (blkdev.h) */

#define BIO_RW_AHEAD	1	/* Must match FAILFAST in req flags */