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>