mips: fix syscall_get_nr

commit 85cc91e2ba4262a602ec65e2b76c4391a9e60d3d 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 201766a20e30f ("ptrace: add
PTRACE_GET_SYSCALL_INFO request") and detected by strace test suite.

Link: strace/strace#235
Fixes: c2d9f17 ("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>

arch/mips/include/asm/syscall.h

@@ -29,7 +29,7 @@
 static inline long syscall_get_nr(struct task_struct *task,
 				  struct pt_regs *regs)
 {
-	return current_thread_info()->syscall;
+	return task_thread_info(task)->syscall;
 }
 
 static inline unsigned long mips_get_syscall_arg(unsigned long *arg,