staging/lustre/libcfs: cleanup libcfs_kkuc_msg_put a bit

No need to cast count since it is already ssize_t. No need
to cast payload to const, but need __force instead to avoid
Sparse complaining.

Reported-and-Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Peng Tao <tao.peng@emc.com>
Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>