From: Christoph Hellwig <hch@lst.de>

partition_name() is a variant of __bdevname() that caches results and
returns a pointrer to kmalloc()ed data instead of printing into a buffer.
Due to it's caching it gets utterly confused when the name for a dev_t
changes (can happen easily now with device mapper and probably in the
future with dynamic dev_t users).

It's only used by the raid code and most calls are through a wrapper,
bdev_partition_name() which takes a struct block_device * that maybe be
NULL.

The patch below changes the bdev_partition_name() to call bdevname() if
possible and the other calls where we really have nothing more than a dev_t
to __bdevname.

Btw, it would be nice if someone who knows the md code a bit better than me
could remove bdev_partition_name() in favour of direct calls to bdevname()
where possible - that would also get rid of the returns pointer to string
on stack issue that this patch can't fix yet.