vfio: fix leak with multiprocess

vfio: fix leak with multiprocess

The code checks both rte_mp_request_sync() return code and that the number
of messages in the reply equals 1.  If rte_mp_request_sync() succeeds but
there was more than one message, those messages would get leaked.

Found via code review by Anatoly Burakov of patches that used the vhost
code as a template for using rte_mp_request_sync().

Fixes: 83a73c5fef66 ("vfio: use generic multi-process channel")
Cc: stable@dpdk.org
Reported-by: Anatoly Burakov <anatoly.burakov@intel.com>
Signed-off-by: Jim Harris <james.r.harris@intel.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>