dpif-netdev: Fix dp_netdev_pmd_remove_flow().

After removing a flow from the dpcls classifier there might still be
readers who have access to the flow, until the next grace period.

Setting flow->cr.mask to NULL can cause concurrent readers to crash,
so this commit avoids doing it.

The crash can be reproduced, for example, by invoking an operation
that cause datapath flows to be deleted (such as `ovs-appctl
upcall/enable-megaflows`) while traffic is running.

I think the assignment was intended just as a safety measure to catch
race conditions, and it should be safe to remove.

Here's a stack trace of a possible crash:

Program terminated with signal SIGSEGV, Segmentation fault.
rule=0x7f3ae8006190) at ../lib/dpif-netdev.c:4156
4156            if (OVS_UNLIKELY((value & *maskp++) != *keyp++)) {
(gdb) bt
rule=0x7f3ae8006190) at ../lib/dpif-netdev.c:4156
rules=0x7f3afa3f2e40, cnt=<optimized out>) at ../lib/dpif-netdev.c:4225
(pmd=pmd@entry=0x7f3afa3fc010, packets=packets@entry=0x7f3afa3fa420,
cnt=cnt@entry=32, keys=keys@entry=0x7f3afa3f6428,
batches=batches@entry=0x7f3afa3f4118,
n_batches=n_batches@entry=0x7f3afa3fa3b0)
    at ../lib/dpif-netdev.c:3483
(pmd=pmd@entry=0x7f3afa3fc010, packets=packets@entry=0x7f3afa3fa420,
cnt=<optimized out>, md_is_valid=md_is_valid@entry=false,
port_no=<optimized out>) at ../lib/dpif-netdev.c:3625
cnt=<optimized out>, packets=0x7f3afa3fa420, pmd=0x7f3afa3fc010) at
../lib/dpif-netdev.c:3642
rxq=<optimized out>, port=<optimized out>, port=<optimized out>) at
../lib/dpif-netdev.c:2574
../lib/dpif-netdev.c:2693
../lib/ovs-thread.c:340
pthread_create.c:312
../sysdeps/unix/sysv/linux/x86_64/clone.S:111

Fixes: 361d808dd9e4("flow: Split miniflow's map.")
CC: Jarno Rajahalme <jarno@ovn.org>
Signed-off-by: Daniele Di Proietto <diproiettod@vmware.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>

lib/dpif-netdev.c

@@ -1444,8 +1444,6 @@ dp_netdev_pmd_remove_flow(struct dp_netdev_pmd_thread *pmd,
     struct cmap_node *node = CONST_CAST(struct cmap_node *, &flow->node);
 
     dpcls_remove(&pmd->cls, &flow->cr);
-    flow->cr.mask = NULL;   /* Accessing rule's mask after this is not safe. */
-
     cmap_remove(&pmd->flow_table, node, dp_netdev_flow_hash(&flow->ufid));
     flow->dead = true;