FBSD: plug refcount leak in pioctl

When gop_lookupname_user returns a non-NULL vnode, the vnode came
from afs_GetVCache (by way of afs_lookup) which takes a reference
on the vnode entry.  There's no need to take another spurious
reference here.  The existing code already knows that there's a
reference in place, as there is an AFS_RELE down where FBSD80_ENV
unlocks the vnode if it's locked (that code is also suspicious).

Prior to this patch, things like 'fs flush /path/to/file' would
leak a reference on that cache entry, preventing clean shutdown.

Reviewed-on: http://gerrit.openafs.org/9957
Reviewed-by: Benjamin Kaduk <kaduk@mit.edu>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
(cherry picked from commit 2bbba424ad6728a221688f782b4df90bf6da4a63)

Change-Id: I9862e3bbfe26c4b89c3bb2c3efc27716f8971473