Linux: Only use automount for volume roots

Linux: Only use automount for volume roots

As long as we avoid using directory aliases when crossing
a mount point (at the volume root), we should always get
to a given non root directory with the same dentry.
The mechanism added by commit de381aa0 ("Linux: Make dir
dentry aliases act like symlinks") is therefore only really
necessary for a volume root.

With kernel 4.2 it is not possible to tweak the "total link
count", resulting in ELOOP errors when looking up a path
with 40 or more directories that are being looked up for
the first time.  With this change, only mountpoints will
count against the limit.

Reviewed-on: http://gerrit.openafs.org/11945
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Daria Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 05f64de7d723a8d5430d9b5928c2025838a6fa52)

Change-Id: I16e855c8322174604288b7d440b342951dd3a015
Reviewed-on: http://gerrit.openafs.org/11989
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>