MIPS: Fix watchpoint restoration

Commit f51246efee2b ("MIPS: Get rid of finish_arch_switch().") moved the
__restore_watch() call from finish_arch_switch() (i.e. after resume()
returns) to before the resume() call in switch_to(). This results in
watchpoints only being restored when a task is descheduled, preventing
the watchpoints from being effective most of the time, except due to
chance before the watchpoints are lazily removed.

Fix the call sequence from switch_to() through to
mips_install_watch_registers() to pass the task_struct pointer of the
next task, instead of using current. This allows the watchpoints for the
next (non-current) task to be restored without reintroducing
finish_arch_switch().

Fixes: f51246efee2b ("MIPS: Get rid of finish_arch_switch().")
Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paul Burton <paul.burton@imgtec.com>
Cc: linux-mips@linux-mips.org
Cc: <stable@vger.kernel.org> # 4.3.x-
Patchwork: https://patchwork.linux-mips.org/patch/12726/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

MIPS: Add HARDWARE_WATCHPOINTS definitions and support code.

This is the main support code for the patch. Here we just add the
code, the following patches hook it up.

Signed-off-by: David Daney <ddaney@avtrex.com>
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

create mode 100644 arch/mips/include/asm/watch.h
create mode 100644 arch/mips/kernel/watch.c