libafs: Avoid using changing unixuser ticket data

libafs: Avoid using changing unixuser ticket data

PSetTokens was afs_osi_Alloc'ing after afs_osi_Free'ing the previous
token data. This can sleep, causing tu->stp to be pointing to garbage
while we wait to alloc. Additionally, rxkad_NewClientSecurityObject
can sleep while waiting to alloc memory, and so the given tu->stp
pointer given to it by afs_ConnBySA may be invalid by the time it
actually uses the data.

To fix this, we could implement unixuser locking to ensure mutual
exclusion of these events. However, this implements a more
conservative change for the 1.4 and 1.6 branches. In PSetTokens we
alloc the new memory before we change anything, and in afs_ConnBySA we
make copies of the ticket data before giving it to rxkad. With these
changes, the glock gives us enough serialization to avoid issues with
tu->stp changing underneath us.

This change is specific to 1.4 and 1.6. On the master branch, this
issue is fixed by implementing unixuser locks in change
Idd66d72f716b7e7dc08faa31ae43e9a23639bae3.

Reviewed-on: http://gerrit.openafs.org/4649
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 1465946bb6863430bf0efebd024d394549a8775f)

Change-Id: Icab5176bf685c408447f0f32ad65c5b003299d3d
Reviewed-on: http://gerrit.openafs.org/6345
Reviewed-by: Derrick Brashear <shadow@dementix.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>