Silence suspicious RCU usage warning in ovs_flow_tbl_masks_cache_resize()
by replacing rcu_dereference() with rcu_dereference_ovsl().

In addition, when creating a new datapath, make sure it's configured under
the ovs_lock.

Fixes: 9bf24f59 ("net: openvswitch: make masks cache size configurable")
Reported-by: syzbot+9a8f8bfcc56e8578016c@syzkaller.appspotmail.com
Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
Link: https://lore.kernel.org/r/160439190002.56943.1418882726496275961.stgit@ebuildSigned-off-by: Jakub Kicinski <kuba@kernel.org>