devtmpfs: fix placement of complete() call

Calling complete() from within the __init function is wrong -
theoretically, the init process could proceed all the way to freeing
the init mem before the devtmpfsd thread gets to execute the return
instruction in devtmpfs_setup().

In practice, it seems to be harmless as gcc inlines devtmpfs_setup()
into devtmpfsd(). So the calls of the __init functions init_chdir()
etc. actually happen from devtmpfs_setup(), but the __ref on that one
silences modpost (it's all right, because those calls happen before
the complete()). But it does make the __init annotation of the setup
function moot, which we'll fix in a subsequent patch.

Fixes: bcbacc49

 ("devtmpfs: refactor devtmpfsd()")
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lore.kernel.org/r/20210312103027.2701413-1-linux@rasmusvillemoes.dk
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>