kgdb: fix several kgdb regressions

kgdb core fixes:
- Check to see that mm->mmap_cache is not null before calling
  flush_cache_range(), else on arch=ARM it will cause a fatal
  fault.

- Breakpoints should only be restored if they are in the BP_ACTIVE
  state.

- Fix a typo in comments to "kgdb_register_io_module"

x86 kgdb fixes:
- Fix the x86 arch handler such that on a kill or detach that the
  appropriate cleanup on the single stepping flags gets run.

- Add in the DIE_NMIWATCHDOG call for x86_64

- Touch the nmi watchdog before returning the system to normal
  operation after performing any kind of kgdb operation, else
  the possibility exists to trigger the watchdog.

Signed-off-by: Jason Wessel <jason.wessel@windriver.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>