crypto: arm64: Use x16 with indirect branch to bti_c

The AES code uses a 'br x7' as part of a function called by
a macro. That branch needs a bti_j as a target. This results
in a panic as seen below. Using x16 (or x17) with an indirect
branch keeps the target bti_c.

  Bad mode in Synchronous Abort handler detected on CPU1, code 0x34000003 -- BTI
  CPU: 1 PID: 265 Comm: cryptomgr_test Not tainted 5.8.11-300.fc33.aarch64 #1
  pstate: 20400c05 (nzCv daif +PAN -UAO BTYPE=j-)
  pc : aesbs_encrypt8+0x0/0x5f0 [aes_neon_bs]
  lr : aesbs_xts_encrypt+0x48/0xe0 [aes_neon_bs]
  sp : ffff80001052b730

  aesbs_encrypt8+0x0/0x5f0 [aes_neon_bs]
   __xts_crypt+0xb0/0x2dc [aes_neon_bs]
   xts_encrypt+0x28/0x3c [aes_neon_bs]
  crypto_skcipher_encrypt+0x50/0x84
  simd_skcipher_encrypt+0xc8/0xe0
  crypto_skcipher_encrypt+0x50/0x84
  test_skcipher_vec_cfg+0x224/0x5f0
  test_skcipher+0xbc/0x120
  alg_test_skcipher+0xa0/0x1b0
  alg_test+0x3dc/0x47c
  cryptomgr_test+0x38/0x60

Fixes: 0e89640 ("crypto: arm64 - Use modern annotations for assembly functions")
Cc: <stable@vger.kernel.org> # 5.6.x-
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Suggested-by: Dave P Martin <Dave.Martin@arm.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20201006163326.2780619-1-jeremy.linton@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

arch/arm64/crypto/aes-neonbs-core.S

@@ -788,7 +788,7 @@ SYM_FUNC_START_LOCAL(__xts_crypt8)
 
 0:	mov		bskey, x21
 	mov		rounds, x22
-	br		x7
+	br		x16
 SYM_FUNC_END(__xts_crypt8)
 
 	.macro		__xts_crypt, do8, o0, o1, o2, o3, o4, o5, o6, o7
@@ -806,7 +806,7 @@ SYM_FUNC_END(__xts_crypt8)
 	uzp1		v30.4s, v30.4s, v25.4s
 	ld1		{v25.16b}, [x24]
 
-99:	adr		x7, \do8
+99:	adr		x16, \do8
 	bl		__xts_crypt8
 
 	ldp		q16, q17, [sp, #.Lframe_local_offset]