Abstract out Linux sillyrename function

Abstract out Linux sillyrename function

In order to keep the dcache happy, the Linux client has its own
sillyrename function. Abstract this out from afs_linux_unlink into a
function of its own (afs_linux_sillyrename) so we can make use of it
from other vnodeops.

Change-Id: I298251c400dfc22efb3bacaa72612b28a5409112
Reviewed-on: http://gerrit.openafs.org/1034
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>