LU-5476 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.

Change-Id: I90d6ca290d115eabd9b68c7512c65f7e1fccc752
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: http://review.whamcloud.com/11412
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>