mips: fix syscall_get_nr

mips: fix syscall_get_nr

commit 52390aef66e5e90f960f6d5610d5bfeb00081e5e upstream.

The implementation of syscall_get_nr on mips used to ignore the task
argument and return the syscall number of the calling thread instead of
the target thread.

The bug was exposed to user space by commit 581c64a5d518f ("ptrace: add
PTRACE_GET_SYSCALL_INFO request") and detected by strace test suite.

Link: https://github.com/strace/strace/issues/235
Fixes: ec8f76e2e5f1 ("MIPS: Fix syscall_get_nr for the syscall exit tracing.")
Cc: <stable@vger.kernel.org> # v3.19+
Co-developed-by: Dmitry V. Levin <ldv@strace.io>
Signed-off-by: Dmitry V. Levin <ldv@strace.io>
Signed-off-by: Elvira Khabirova <lineprinter0@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>