LU-2724 ptlrpc: skip NULL obd_svc_stats in lprocfs_rd_import()

In lprocfs_rd_import() don't print trivial obd_svc_stats if they were
not allocated for this import. Add a general proc file read check to
sanity.

Signed-off-by: John L. Hammond <john.hammond@intel.com>
Change-Id: Ic94056eb4f02d71491cdaf948cbe27b82de2153d
Reviewed-on: http://review.whamcloud.com/5234
Reviewed-by: Bobi Jam <bobijam@gmail.com>
Tested-by: Hudson
Reviewed-by: Li Wei <wei.g.li@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>