staging/lustre/llite: check for integer overflow in hsm user request

Check to make sure total size of request does not overflow when
calculated.  Return -1 from hur_len() if it does overflow.

Signed-off-by: Nathaniel Clark <nathaniel.l.clark@intel.com>
Reviewed-on: http://review.whamcloud.com/10615
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4984
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>