crypto: aesni - check walk.nbytes instead of err

In the code for xts_crypt(), we check for the err value returned by
skcipher_walk_virt() and return from the function if it is non zero.
However, skcipher_walk_virt() can set walk.nbytes to 0, which would cause
us to call kernel_fpu_begin(), and then skip the kernel_fpu_end() call.

This patch checks for the walk.nbytes value instead, and returns if
walk.nbytes is 0. This prevents us from calling kernel_fpu_begin() in
the first place and also covers the case of having a non zero err value
returned from skcipher_walk_virt().
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Shreyansh Chouhan <chouhan.shreyansh630@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

arch/x86/crypto/aesni-intel_glue.c

@@ -866,7 +866,7 @@ static int xts_crypt(struct skcipher_request *req, bool encrypt)
 		req = &subreq;
 
 		err = skcipher_walk_virt(&walk, req, false);
-		if (err)
+		if (!walk.nbytes)
 			return err;
 	} else {
 		tail = 0;