missing helpers: bdev_unhash(), bdev_drop()

bdev_unhash(): make block device invisible to lookups by device number
bdev_drop(): drop reference to associated inode.

Both are internal, for use by genhd and partition-related code - similar
to bdev_add().  The logics in there (especially the lifetime-related
parts of it) ought to be cleaned up, but that's a separate story; here
we just encapsulate getting to associated inode.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>