Linux: don't oops on writeback if vcache has no stashed credentials

Linux: don't oops on writeback if vcache has no stashed credentials

When doing writeback, use current credentials if we can't find
stashed credentials in the vcache entry.
This is normally stashed in afs_open, so it's not clear how we
get to that point, but that's not a reason to trigger an oops.

Change-Id: I60937e3f9e360e68ea84205570c05cabb47ef08e
Reviewed-on: http://gerrit.openafs.org/1063
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>