devtmpfs: factor out setup part of devtmpfsd()

Factor out the setup part of devtmpfsd() to make it a bit easier to
see that we always call setup_done() exactly once (provided of course
the kthread is succesfully created).

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20200115184154.3492-3-linux@rasmusvillemoes.dk


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>