Linux: cache bypass: warning fixes in afs_pioctl.c

Linux: cache bypass: warning fixes in afs_pioctl.c

Fix two minor warnings in afs_pioctl.c:

- gcc complains that threshold may get used uninitialized.  the
warning looks bogus, but initialize it to keep gcc happy.

- PSetCachingBlkSize is declared but never defined.

Change-Id: I3ac84d665f60ba51fef2d52f2bd8d90e0a7bc3b3
Reviewed-on: http://gerrit.openafs.org/2322
Reviewed-by: Matt Benjamin <matt@linuxbox.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>