staging/lustre/llite: Fix integer overflow in ll_fid2path

Reported by Dan Carpenter <dan.carpenter@oracle.com>

outsize = sizeof(*gfout) + gfin->gf_pathlen;

Where outsize is int and gf_pathlen is u32 from userspace
can lead to integer overflowwhere outsize is some small number
less than sizeof(*gfout)

Add a check for pathlen to be of sensical size.

Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/11412
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5476


Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>