devtmpfs: factor out common tail of devtmpfs_{create,delete}_node

There's some common boilerplate in devtmpfs_{create,delete}_node, put
that in a little helper.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20200115184154.3492-6-linux@rasmusvillemoes.dk


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