afs: Leave cellnum alone for explicit mtpt cell

afs: Leave cellnum alone for explicit mtpt cell

When a mountpoint is given an explicit cell, don't alter cellnum.
Cellnum represents the cell for the parent, and is used for
determining whether or not we're crossing a cell boundary.

Previously, this code forced the mount point to always be treated as
foreign (for a mountpoint prefixed with a cell name), or to always be
treated as local (for a mountpoint prefixed with a cell number).

Change-Id: I7b98ccc94bf61dc3d7be0b7d6b54019cd797e884
Reviewed-on: http://gerrit.openafs.org/6051
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>