afs: Fix vlserver record corruption

Because I made the afs_call struct share pointers to an afs_server object
and an afs_vlserver object to save space, afs_put_call() calls
afs_put_server() on afs_vlserver object (which is only meant for the
afs_server object) because it sees that call->server isn't NULL.

This means that the afs_vlserver object gets unpredictably and randomly
modified, depending on what config options are set (such as lockdep).

Fix this by getting rid of the union and having two non-overlapping
pointers in the afs_call struct.

Fixes: ffba718e ("afs: Get rid of afs_call::reply[]")
Signed-off-by: David Howells <dhowells@redhat.com>