devtmpfs: refactor devtmpfsd()

Split the main worker loop into a separate function.  This allows
devtmpfsd_setup to be marked __init, which will allows us to call
__init routines for the setup work.  devtmpfѕ itself needs a __ref
marker for that to work, and a comment explaining why it works.

Signed-off-by: Christoph Hellwig <hch@lst.de>