In the current code, gdb can set the watchpoint successfully through
ptrace interface, but watchpoint will not be triggered.

When debugging the following code using gdb.

lihui@bogon:~$ cat test.c
  #include <stdio.h>
  int a = 0;
  int main()
  {
	a = 1;
	printf("a = %d\n", a);
	return 0;
  }
lihui@bogon:~$ gcc -g test.c -o test
lihui@bogon:~$ gdb test
...
(gdb) watch a
...
(gdb) r
...
a = 1
[Inferior 1 (process 4650) exited normally]

No watchpoints were triggered, the root causes are:

1. Kernel uses perf_event and hw_breakpoint framework to control
   watchpoint, but the perf_event corresponding to watchpoint is
   not enabled. So it needs to be enabled according to MWPnCFG3
   or FWPnCFG3 PLV bit field in ptrace_hbp_set_ctrl(), and privilege
   is set according to the monitored addr in hw_breakpoint_control().
   Furthermore, add a judgment in ptrace_hbp_set_addr() to ensure
   kernel-space addr cannot be monitored in user mode.

2. The global enable control for all watchpoints is the WE bit of
   CSR.CRMD, and hardware sets the value to 0 when an exception is
   triggered. When the ERTN instruction is executed to return, the
   hardware restores the value of the PWE field of CSR.PRMD here.
   So, before a thread containing watchpoints be scheduled, the PWE
   field of CSR.PRMD needs to be set to 1. Add this modification in
   hw_breakpoint_control().

All changes according to the LoongArch Reference Manual:
https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#control-and-status-registers-related-to-watchpoints
https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#basic-control-and-status-registers

With this patch:

lihui@bogon:~$ gdb test
...
(gdb) watch a
Hardware watchpoint 1: a
(gdb) r
...
Hardware watchpoint 1: a

Old value = 0
New value = 1
main () at test.c:6
6		printf("a = %d\n", a);
(gdb) c
Continuing.
a = 1
[Inferior 1 (process 775) exited normally]

Cc: stable@vger.kernel.org
Signed-off-by: Hui Li <lihui@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>