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).

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>
(cherry picked from commit e14dec55e6600edb60ce5184b4ab1f646c68947b)

Change-Id: I8d2b378a95533aabd65e7121000ce34238d728b7
Reviewed-on: http://gerrit.openafs.org/6123
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>