bootup: move 'usermodehelper_enable()' a little earlier

Commit d5767c53 ("bootup: move 'usermodehelper_enable()' to the end
of do_basic_setup()") moved 'usermodehelper_enable()' to end of
do_basic_setup() to after the initcalls.  But then I get failed to let
uvesafb work on my computer, and lose the splash boot.

So maybe we could start usermodehelper_enable a little early to make
some task work that need eary init with the help of user mode.

[ I would *really* prefer that initcalls not call into user space - even
  the real 'init' hasn't been execve'd yet, after all! But for uvesafb
  it really does look like we don't have much choice.

  I considered doing this when we mount the root filesystem, but
  depending on config options that is in multiple places.  We could do
  the usermode helper enable as a rootfs_initcall()..

  So I'm just using wang yanqing's trivial patch.  It's not wonderful,
  but it's simple and should work.  We should revisit this some day,
  though.      - Linus ]

Signed-off-b...