FBSD: Switch the dummy 'data' for mount(2)

The mount(2) API takes a void*, but 'rn' is const char*, which
is const-incorrect.  Our vfs_cmount implementation ignores the 'data'
parameter, but upstream's kernel mount(2) implementation did
have a NULL check until r158611 (in the 6.1 or 7.0 timeframe),
so leave that comment for now.

Arguably we should be using nmount(2) instead of mount(2) anyway,
but leave that for a separate patch.

Change-Id: I22fd85c2f1a32aa849e182d263de119a953690d2
Reviewed-on: http://gerrit.openafs.org/10821
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@your-file-system.com>