crypto: skcipher - fix crash in skcipher_walk_aead()

The new skcipher_walk_aead() may crash in the following way due to
the walk flag SKCIPHER_WALK_PHYS not being cleared at the start of the
walk:

Unable to handle kernel NULL pointer dereference at virtual address 00000001
[..]
Internal error: Oops: 96000044 [#1] PREEMPT SMP
[..]
PC is at skcipher_walk_next+0x208/0x450
LR is at skcipher_walk_next+0x1e4/0x450
pc : [<ffff2b93b7104e20>] lr : [<ffff2b93b7104dfc>] pstate: 40000045
sp : ffffb925fa517940
[...]
[<ffff2b93b7104e20>] skcipher_walk_next+0x208/0x450
[<ffff2b93b710535c>] skcipher_walk_first+0x54/0x148
[<ffff2b93b7105664>] skcipher_walk_aead+0xd4/0x108
[<ffff2b93b6e77928>] ccm_encrypt+0x68/0x158

So clear the flag at the appropriate time.
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

crypto/skcipher.c

@@ -506,6 +506,8 @@ int skcipher_walk_aead(struct skcipher_walk *walk, struct aead_request *req,
 	struct crypto_aead *tfm = crypto_aead_reqtfm(req);
 	int err;
 
+	walk->flags &= ~SKCIPHER_WALK_PHYS;
+
 	scatterwalk_start(&walk->in, req->src);
 	scatterwalk_start(&walk->out, req->dst);