*	new functions - bd_claim(bdev, holder) and bd_release(bdev).
	bd_claim(bdev, holder) fails is device is already claimed by
	somebody else; bd_release(bdev) gives device up.

*	get_sb_bdev() claims device for fs_type; it means that we don't need
	to look through entire least of superblocks anymore - just through
	the list of superblocks belonging to that type (i.e. the same thing
	we do for non-block filesystems; that will allow to merge quite a
	bit of code afterwards).

*	sys_swapon claims device for itself;  free exclusion with mounting,
	end of problems with bogus set_blocksize().

*	is_mounted() and is_swap_partition() are gone - what we actually
	wanted was "try to claim device for ourselves".  Which we can do
	now - without races inherent to is_mounted()/is_swap_partition().

*	RAID lock_rdev() claims device for itself.  I.e. we get rid of
	is_mounted() in there (BTW, is_swap_partition() was missing) and
	we get protection both ways - not only RAID won't take an already
	mounted device, but mount won't stomp on a device claimed by RAID.

There are other places that would benefit from the same (e.g. ext3 with
external journal almost definitely wants to claim device for itself).

Notice that it's a cooperative thing - neither open() nor raw device stuff
claim the block device, so they don't care if device is mounted, etc.  So
we don't break fsck and friends - exclusion is between those who know that
they want that exclusion.