LINUX: Dir follow_link should set LAST_BIND

LINUX: Dir follow_link should set LAST_BIND

For our faux-symlink directory follow_link operation, we leave the
given nameidata struct with an invalid 'last' component. That is,
nd->last is not changed or set to anything meaningful.

Usually the callers of our follow_link op do not care about the last
component of the nameidata. However, at least one caller does: the
caller near the do_link label in open_namei(). This is called during
processing for O_CREAT operations on symlinks, and since our
directories look like symlinks, it gets called. It tries to use
nd->last to look up the last component of the dereferenced path (so it
can try to create it, as necessary), but since our nd->last is not
set, this will not work.

Specifically, our nd->last.name is not pointing into the names cache,
so the subsequent putname/__putname on it will corrupt the names
cache. However, even if this were not a problem, the actual contents
of the last component do not seem meaningful so this would probably
result in incorrect behavior anyway.

To avoid all of this, set nd->last_type to LAST_BIND, so any callers
know that the last component of the given nd is not valid, and we are
pointing directly to the target component with a dentry.

Change-Id: I9cebc3b63ae7a2410295392a08aa8fc738549234
Reviewed-on: http://gerrit.openafs.org/8489
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>