xtensa: use actual syscall number in do_syscall_trace_leave

Syscall may alter pt_regs structure passed to it, resulting in a
mismatch between syscall entry end syscall exit entries in the ftrace.
Temporary restore syscall field of the pt_regs for the duration of
do_syscall_trace_leave.
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/kernel/entry.S

@@ -1860,6 +1860,8 @@ ENTRY(system_call)
 	l32i	a7, a2, PT_SYSCALL
 
 1:
+	s32i	a7, a1, 4
+
 	/* syscall = sys_call_table[syscall_nr] */
 
 	movi	a4, sys_call_table
@@ -1893,8 +1895,12 @@ ENTRY(system_call)
 	retw
 
 1:
+	l32i	a4, a1, 4
+	l32i	a3, a2, PT_SYSCALL
+	s32i	a4, a2, PT_SYSCALL
 	mov	a6, a2
 	call4	do_syscall_trace_leave
+	s32i	a3, a2, PT_SYSCALL
 	retw
 
 ENDPROC(system_call)