armv8_crypto: fix recursive fpu_kern_enter call

Now armv8_crypto is using FPU_KERN_NOCTX, this results in a kernel panic
in armv8_crypto.c:armv8_crypto_cipher_setup:

    panic: recursive fpu_kern_enter while in PCB_FP_NOSAVE state

This is because in armv8_crypto.c:armv8_crypto_cipher_process,
directly after calling fpu_kern_enter() a call is made to
armv8_crypto_cipher_setup(), resulting in nested calls to
fpu_kern_enter() without the required fpu_kern_leave() in between.

Move fpu_kern_enter() in armv8_crypto_cipher_process() after the
call to armv8_crypto_cipher_setup() to resolve this.

Reviewed by:	markj, andrew
Fixes: 6485286 ("armv8_crypto: Switch to using FPU_KERN_NOCTX")
Sponsored by: Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D41671

sys/crypto/armv8/armv8_crypto.c

@@ -336,15 +336,15 @@ armv8_crypto_cipher_process(struct armv8_crypto_session *ses,
 		crypto_cursor_copy(&fromc, &toc);
 	}
 
-	fpu_kern_enter(curthread, NULL, FPU_KERN_NORMAL | FPU_KERN_NOCTX);
-
 	if (crp->crp_cipher_key != NULL) {
 		armv8_crypto_cipher_setup(ses, csp, crp->crp_cipher_key,
 		    csp->csp_cipher_klen);
 	}
 
 	crypto_read_iv(crp, iv);
 
+	fpu_kern_enter(curthread, NULL, FPU_KERN_NORMAL | FPU_KERN_NOCTX);
+
 	error = 0;
 	switch (csp->csp_cipher_alg) {
 	case CRYPTO_AES_CBC: