crypto: arm64/aes-blk - fix and move skcipher_walk_done out of kernel_neon_begin, _end

In a arm64 server(QDF2400),I met a similar might-sleep warning as [1]:
[    7.019116] BUG: sleeping function called from invalid context at
./include/crypto/algapi.h:416
[    7.027863] in_atomic(): 1, irqs_disabled(): 0, pid: 410, name:
cryptomgr_test
[    7.035106] 1 lock held by cryptomgr_test/410:
[    7.039549]  #0:         (ptrval) (&drbg->drbg_mutex){+.+.}, at:
drbg_instantiate+0x34/0x398
[    7.048038] CPU: 9 PID: 410 Comm: cryptomgr_test Not tainted
4.17.0-rc6+ #27
[    7.068228]  dump_backtrace+0x0/0x1c0
[    7.071890]  show_stack+0x24/0x30
[    7.075208]  dump_stack+0xb0/0xec
[    7.078523]  ___might_sleep+0x160/0x238
[    7.082360]  skcipher_walk_done+0x118/0x2c8
[    7.086545]  ctr_encrypt+0x98/0x130
[    7.090035]  simd_skcipher_encrypt+0x68/0xc0
[    7.094304]  drbg_kcapi_sym_ctr+0xd4/0x1f8
[    7.098400]  drbg_ctr_update+0x98/0x330
[    7.102236]  drbg_seed+0x1b8/0x2f0
[    7.105637]  drbg_instantiate+0x2ac/0x398
[    7.109646]  drbg_kcapi_seed+0xbc/0x188
[    7.113482]  crypto_rng_reset+0x4c/0xb0
[    7.117319]  alg_test_drbg+0xec/0x330
[    7.120981]  alg_test.part.6+0x1c8/0x3c8
[    7.124903]  alg_test+0x58/0xa0
[    7.128044]  cryptomgr_test+0x50/0x58
[    7.131708]  kthread+0x134/0x138
[    7.134936]  ret_from_fork+0x10/0x1c

Seems there is a bug in Ard Biesheuvel's commit.
Fixes: 68338174 ("crypto: arm64/aes-blk - move kernel mode neon
en/disable into loop")

[1] https://www.spinics.net/lists/linux-crypto/msg33103.html

Signed-off-by: jia.he@hxt-semitech.com
Acked-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: <stable@vger.kernel.org> # 4.17
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

arch/arm64/crypto/aes-glue.c

@@ -223,8 +223,8 @@ static int ctr_encrypt(struct skcipher_request *req)
 		kernel_neon_begin();
 		aes_ctr_encrypt(walk.dst.virt.addr, walk.src.virt.addr,
 				(u8 *)ctx->key_enc, rounds, blocks, walk.iv);
-		err = skcipher_walk_done(&walk, walk.nbytes % AES_BLOCK_SIZE);
 		kernel_neon_end();
+		err = skcipher_walk_done(&walk, walk.nbytes % AES_BLOCK_SIZE);
 	}
 	if (walk.nbytes) {
 		u8 __aligned(8) tail[AES_BLOCK_SIZE];