devtmpfs: simplify initialization of mount_dev

Avoid a bit of ifdeffery by using the IS_ENABLED() helper.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20200115184154.3492-4-linux@rasmusvillemoes.dk


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