autofs4: add miscellaneous device for ioctls

autofs4: add miscellaneous device for ioctls

Add a miscellaneous device to the autofs4 module for routing ioctls.  This
provides the ability to obtain an ioctl file handle for an autofs mount
point that is possibly covered by another mount.

The actual problem with autofs is that it can't reconnect to existing
mounts.  Immediately one things of just adding the ability to remount
autofs file systems would solve it, but alas, that can't work.  This is
because autofs direct mounts and the implementation of "on demand mount
and expire" of nested mount trees have the file system mounted on top of
the mount trigger dentry.

To resolve this a miscellaneous device node for routing ioctl commands to
these mount points has been implemented in the autofs4 kernel module and a
library added to autofs.  This provides the ability to open a file
descriptor for these over mounted autofs mount points.

Please refer to Documentation/filesystems/autofs4-mount-control.txt for a
discussion of the problem, implementation alternatives considered and a
description of the interface.

[akpm@linux-foundation.org: coding-style fixes]
[akpm@linux-foundation.org: build fix]
Signed-off-by: Ian Kent <raven@themaw.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>