Update the thread_info::syscall field when registers are modified via
ptrace to change or cancel the system call being entered.

This is important to allow seccomp and the syscall entry and exit trace
events to observe the new syscall number changed by the normal ptrace
hook or seccomp. That includes allowing seccomp's recheck of the system
call number after SECCOMP_RET_TRACE to notice if the syscall is changed
to a denied one, which happens in seccomp since commit ce6526e8
("seccomp: recheck the syscall after RET_TRACE") in v4.8.

In the process of doing this, the logic to determine whether an indirect
system call is in progress (i.e. the O32 ABI's syscall()) is abstracted
into mips_syscall_is_indirect(), and a new mips_syscall_update_nr() is
used to update the thread_info::syscall based on the register state.

The following ptrace operations are updated:
 - PTRACE_SETREGS (ptrace_setregs()).
 - PTRACE_SETREGSET with NT_PRSTATUS (gpr32_set() and gpr64_set()).
 - PTRACE_POKEUSR with 2/v0 or 4/a0 for indirect syscall
   ([compat_]arch_ptrace()).

Fixes: c2d9f177 ("MIPS: Fix syscall_get_nr for the syscall exit tracing.")
Signed-off-by: James Hogan <jhogan@kernel.org>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Lars Persson <larper@axis.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/16995/