vhost: Cache qsz_mask instead of qsz in vhost_user_vring_t

vhost: Cache qsz_mask instead of qsz in vhost_user_vring_t

In the data path, we grab qsz from vhost_user_vring_t to compute
qsz_mask and store it in a stack variable to use on many occasions.
We never use qsz for any meaningful purpose. It is more useful to
cache qsz_mask in vhost_user_vring_t to avoid the needless computation
in the data path.

Change-Id: Idf4d94a9754d5c75c899f1f4f59602275b9904a6
Signed-off-by: Steven <sluong@cisco.com>