Stop using storage after free

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Kevin Coffman <kwc@citi.umich.edu>
The previous patch seems to expose a use after free bug in dirscancb. At
least, I could reliably reproduce a segfault by doing a bunch of mounts
and then unmounting them all. The code uses the following list macro:

      TAILQ_FOREACH(ic, icq, ic_next) {

...to iterate over all of the ic entries and clean up any that no longer
have a corresponding directory in rpc_pipefs. This macro unrolls into:

      for(ic=icq->tqh_first; ic != NULL; ic=ic->ic_next.tqe_next) {

...but within this loop, we can free ic, and then the for loop can trip
over that when it tries to do the iteration. The attached patch works
around this by not using the TAILQ_FOREACH macro and saving off the
tqe_next pointer prior to the free.

Again, this was tested on a patched 1.0.6, but the 1.0.10 code is very
similar, and I think the problem exists there as well.
Signed-off-by: Neil Brown <neilb@suse.de>