Catch up to FreeBSD KPI for vfs_cmount

Almost a year ago, mckusick changed the VFS KPI/KBI for the cmount
VFS operation, making the flags argument a 64-bit quantity.
Introduce appropriate conditionals for our prototype of afs_cmount
for the change on the 10.x and 9.x branches.

Change-Id: Ia0b8759f027e64f98f087daabbf1a51030171f21
Reviewed-on: http://gerrit.openafs.org/8907
Reviewed-by: Garrett Wollman <wollman@csail.mit.edu>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>