autofs: rename 'autofs' module back to 'autofs4'

It turns out that systemd has a bug: it wants to load the autofs module
early because of some initialization ordering with udev, and it doesn't
do that correctly.  Everywhere else it does the proper "look up module
name" that does the proper alias resolution, but in that early code, it
just uses a hardcoded "autofs4" for the module name.

The result of that is that as of commit a2225d93 ("autofs: remove
left-over autofs4 stubs"), you get

    systemd[1]: Failed to insert module 'autofs4': No such file or directory

in the system logs, and a lack of module loading.  All this despite the
fact that we had very clearly marked 'autofs4' as an alias for this
module.

What's so ridiculous about this is that literally everything else does
the module alias handling correctly, including really old versions of
systemd (that just used 'modprobe' to do this), and even all the other
systemd module loading code.

Only that special systemd early module load code is broken, hardcoding
the module names for not just 'autofs4', but also "ipv6", "unix",
"ip_tables" and "virtio_rng".  Very annoying.

Instead of creating an _additional_ separate compatibility 'autofs4'
module, just rely on the fact that everybody else gets this right, and
just call the module 'autofs4' for compatibility reasons, with 'autofs'
as the alias name.

That will allow the systemd people to fix their bugs, adding the proper
alias handling, and maybe even fix the name of the module to be just
"autofs" (so that they can _test_ the alias handling).  And eventually,
we can revert this silly compatibility hack.

See also

    https://github.com/systemd/systemd/issues/9501
    https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=902946



for the systemd bug reports upstream and in the Debian bug tracker
respectively.

Fixes: a2225d93 ("autofs: remove left-over autofs4 stubs")
Reported-by: Ben Hutchings <ben@decadent.org.uk>
Reported-by: Michael Biebl <biebl@debian.org>
Cc: Ian Kent <raven@themaw.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>