--examine-bitmap: give useful message if no bitmap found on md array.

--examine-bitmap: give useful message if no bitmap found on md array.

The bitmap is stored on member devices, not on the array, so
--examine-bitmap should be given the member device.
If --examine-bitmap is given an array, and it doesn't have a bitmap
on it (i.e. it isn't a member of some other array), then that
is probably a usage error, so print a helpful message.

Signed-off-by: NeilBrown <neilb@suse.de>