Make namei convertROtoRW'd volumes usable

Make namei convertROtoRW'd volumes usable

Right now, if you convertROtoRW a volume on namei, the converted volume
appears to need a salvage before it is usable, and the header of the old
(now empty) RO volume is kept around. Fix this:

 -- Set inUse = 0 on the converted volume, so the fileserver will be
    able to attach the volume when we give it back

 -- Unlink the RO header file, instead of trying to unlink the
    VI_VOLINFO file twice

 -- Log the actual error code (errno) in the error message for the last
    unlink

Reviewed-on: http://gerrit.openafs.org/591
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit 906403c6d770026fd9da606ea468e47ee911f896)
Reviewed-on: http://gerrit.openafs.org/613