x86/kgdb: Return 0 from kgdb_arch_set_breakpoint()

err must be nonzero in order to reach text_poke(), which caused kgdb to
fail to set breakpoints:

  (gdb) break __x64_sys_sync
  Breakpoint 1 at 0xffffffff81288910: file ../fs/sync.c, line 124.
  (gdb) c
  Continuing.
  Warning:
  Cannot insert breakpoint 1.
  Cannot access memory at address 0xffffffff81288910

  Command aborted.

Fixes: 86a2205 ("x86/kgdb: Avoid redundant comparison of patched code")
Signed-off-by: Matt Mullins <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Reviewed-by: Nadav Amit <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Christophe Leroy <[email protected]>
Cc: Daniel Thompson <[email protected]>
Cc: Douglas Anderson <[email protected]>
Cc: "Gustavo A. R. Silva" <[email protected]>
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: "Peter Zijlstra (Intel)" <[email protected]>
Cc: Rick Edgecombe <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

Author: mokomull

arch/x86/kernel/kgdb.c

@@ -758,7 +758,7 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
 		       BREAK_INSTR_SIZE);
 	bpt->type = BP_POKE_BREAKPOINT;
 
-	return err;
+	return 0;
 }
 
 int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)