staging: lustre: potential underflow in libcfs_kkuc_group_add()

staging: lustre: potential underflow in libcfs_kkuc_group_add()

My static checker says that "group" is a user controlled number that can
be negative leading to an array underflow.  I have looked at it, and I'm
not an expert enough in lustre to say for sure if it is really a bug.
Anyway, it's simple enough to make the variable unsigned which pleases
the static checker and makes it easier to audit.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>