Linux: Check for multiple silly renames

Linux: Check for multiple silly renames

We don't want to do multiple silly renames of the same dcache entry,
so add a check for that occuring, and just return EBUSY if we're
trying to do so.

Change-Id: Ic1cb4061d89bf87926995162f1ac410375bddcb5
Reviewed-on: http://gerrit.openafs.org/1035
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>