devtmpfs: initify a bit

devtmpfs: initify a bit

devtmpfs_mount() is only called from prepare_namespace() in
init/do_mounts.c, which is an __init function, so devtmpfs_mount() can
also be moved to .init.text.

Then the mount_dev static variable is only referenced from __init
functions (devtmpfs_mount and its initializer function mount_param).

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20200115184154.3492-5-linux@rasmusvillemoes.dk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>