[PATCH] debugfs: Add debugfs_create_blob() helper for exporting binary data

I wanted to export a binary blob via debugfs, and although it was pretty easy
it seems like it'd be easier if there was a helper for it. It's a pity we need
the wrapper struct but I can't see a cleaner way to do it.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>