[PATCH] invalidate_device()/check_disk_change() fixes

[PATCH] invalidate_device()/check_disk_change() fixes

	* bogus calls of invalidate_buffers() gone from floppy_open()
	* invalidate_buffers() killed.

	* new helper - __invalidate_device(bdev, do_sync).  invalidate_device()
is calling it.

	* fixed races between floppy_open()/floppy_open and
floppy_open()/set_geometry():
	a) floppy_open()/floppy_release() is done under a semaphore.  That
closes the races between simultaneous open() on /dev/fd0foo and /dev/fd0bar.
	b) pointer to struct block_device is kept as long as floppy is
opened (per-drive, non-NULL when number of openers is non-zero, does not
contribute to block_device refcount).
	c) set_geometry() grabs the same semaphore and invalidates the
devices directly instead of messing with setting fake "it had changed"
and calling __check_disk_change().

	* __check_disk_change() killed - no remaining callers
	* full_check_disk_change() killed - ditto.