md: deprecate bitmap file support

The support for bitmaps on files is a very bad idea abusing various kernel
APIs, and fundamentally requires the file to not be on the actual array
without a way to check that this is actually the case.  Add a deprecation
warning to see if we might be able to eventually drop it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230615064840.629492-12-hch@lst.de