RISC-V: Add fast call path of crash_kexec()

Currently, almost all archs (x86, arm64, mips...) support fast call
of crash_kexec() when "regs && kexec_should_crash()" is true. But
RISC-V not, it can only enter crash system via panic(). However panic()
doesn't pass the regs of the real accident scene to crash_kexec(),
it caused we can't get accurate backtrace via gdb,
	$ riscv64-linux-gnu-gdb vmlinux vmcore
	Reading symbols from vmlinux...
	[New LWP 95]
	#0  console_unlock () at kernel/printk/printk.c:2557
	2557                    if (do_cond_resched)
	(gdb) bt
	#0  console_unlock () at kernel/printk/printk.c:2557
	#1  0x0000000000000000 in ?? ()

With the patch we can get the accurate backtrace,
	$ riscv64-linux-gnu-gdb vmlinux vmcore
	Reading symbols from vmlinux...
	[New LWP 95]
	#0  0xffffffe00063a4e0 in test_thread (data=<optimized out>) at drivers/test_crash.c:81
	81             *(int *)p = 0xdead;
	(gdb)
	(gdb) bt
	#0  0xffffffe00064d5c0 in test_thread (data=<optimized out>) at drivers/test_crash.c:81
	#1  0x0000000000000000 in ?? ()

Test code to produce NULL address dereference in test_crash.c,
	void *p = NULL;
	*(int *)p = 0xdead;
Reviewed-by: Guo Ren <guoren@kernel.org>
Tested-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220606082308.2883458-1-xianting.tian@linux.alibaba.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/kernel/traps.c

@@ -16,6 +16,7 @@
 #include <linux/mm.h>
 #include <linux/module.h>
 #include <linux/irq.h>
+#include <linux/kexec.h>
 
 #include <asm/asm-prototypes.h>
 #include <asm/bug.h>
@@ -44,6 +45,9 @@ void die(struct pt_regs *regs, const char *str)
 
 	ret = notify_die(DIE_OOPS, str, regs, 0, regs->cause, SIGSEGV);
 
+	if (regs && kexec_should_crash(current))
+		crash_kexec(regs);
+
 	bust_spinlocks(0);
 	add_taint(TAINT_DIE, LOCKDEP_NOW_UNRELIABLE);
 	spin_unlock_irq(&die_lock);