tracing/syscalls: Fix typo in SYSCALL_DEFINE0

The struct syscall_metadata variable name in SYSCALL_DEFINE0
should be __syscall_meta__##sname instead of __syscall_meta_##sname
to match the name that is in SYSCALL_DEFINE1/2/3/4/5/6.

This error causes event_enter_##sname->data to point to the wrong
location, which causes syscalls which are defined by SYSCALL_DEFINE0()
not to be traced.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
LKML-Reference: <4B273D2E.1010807@cn.fujitsu.com>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>