Linux: Make dir dentry aliases act like symlinks

Currently, we try to invalidate other dentries that exist for a
particular dir inode when we look up a dentry. This is so we try to
avoid duplicate dentries for a directory, which Linux does not like
(you cannot have hardlinks to a dir).

If we cannot invalidate the other aliases (because they are being
used), right now we just return the alias. This can make it very easy
to panic the client, due to the sanity checks Linux performs when dong
things like 'rmdir'. If we do something like this:

mkdir dir1
fs mkm dir1/mtpt vol
mkdir dir1/mtpt/dir2
fs mkm dir1/mtpt/dir2/mtpt2 vol
cd dir1/mtpt
rmdir dir2/mtpt2

For the 'rmdir', we will lookup 'mtpt2'. Since 'mtpt' and 'mtpt2'
are mountpoints for the same volume, their dentries point to the same
directory inode. So when we lookup 'mtpt2', we will try to invalidate
the other dentry, but we cannot do that since it is the cwd. So we
return the alias dentry (for 'mtpt'). The Linux VFS layer then does a
sanity check for the rmdir operation, checking that the child dentry's
parent inode is the same as the inode we're performing the rmdir for.
Since the dentry we returned was for 'mtpt', whose parent is 'dir1',
and the actual dir we're performing the rmdir for is 'dir2', this
sanity check fails and we BUG.

To avoid this, make the dentry alias act like a symlink when we
encounter an uninvalidateable dentry alias. That is, we allow multiple
dentry aliases for a directory, however, when the dentry aliases are
actually used, we redirect to a common dentry (via d_automount where
possible, and follow_link elsewhere).

This means that such mountpoints will behave similarly to symlinks, in
that we 'point' to a specific mountpoint dentry. This means that if we
have multiple different ways to get to the same volume, and all are
accessed at the same time, all but one of those mountpoints will
behave like symlinks, pointing to the same mountpoint. So, the '..'
entries for each path will all point to the parent dir of one
mountpoint, meaning that the '..' entry will be "wrong", but for most
cases it will still be correct.

In order to try to make the 'target', pointed-to directory consistent,
we add a new field to struct vcache: target_link. This points to the
dentry we should redirect to, whenever that vcache is referenced. To
avoid (possibly not-feasibly-solvable) problems with refcounting, this
pointer is not actually a reference to the target dentry, but just
serves as a pointer to compare to.

FIXES 130273

Reviewed-on: http://gerrit.openafs.org/7741
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Tested-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit de381aa0d39e88a1ca0c8ccbb2471c5cad5a964c)

Change-Id: I68e1f0a3734f56db66e5fee3571597b5eb17eef8
Reviewed-on: http://gerrit.openafs.org/7951
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
LINUX: Do not lookup immediately recursive mtpts

On Linux, having a mountpoint in a volume root that points to the same
volume can cause serious problems. By 'immediately recursive', I mean
a situation like the following:

fs mkm mtpt vol
fs mkm mtpt/mtpt vol

If there are multiple dentry aliases for the directory (which is
possible if the directory is a mountpoint), an 'rmdir' on the
recursive mountpoint can cause the client to deadlock. Since the
'rmdir' code path in Linux locks the parent directory inode to perform
the rmdir, and locks the child directory inode after performing a
couple of sanity checks. For an immediately recursive mountpoint,
these two inodes are the same, and so we will deadlock.

Reviewed-on: http://gerrit.openafs.org/7742
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>
(cherry picked from commit d785abd5ec9302bdc1b3c33368246e573e0cd65d)

Change-Id: I531adbf66cf6714e04811356b072b17335980aa3
Reviewed-on: http://gerrit.openafs.org/7952
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>