selftests/ftrace: Drop invalid top-level local in test_ownership

selftests/ftrace: Drop invalid top-level local in test_ownership

test_ownership.tc is sourced by ftracetest under /bin/sh.

The script currently declares mount_point with local at file scope,
which makes /bin/sh abort with "local: not in a function" before the
test can reach the eventfs ownership checks.

Replace the top-level local declaration with a normal shell variable so
kernels that support the gid= tracefs mount option can run the test at
all.

Link: https://lore.kernel.org/r/20260407102613.81419-1-create0818@163.com
Fixes: 8b55572e51805 ("tracing/selftests: Add tracefs mount options test")
Signed-off-by: Cao Ruichuang <create0818@163.com>
Reviewed-by: Steven Rostedt (Google) <rostedt@goodmis.org>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Cc: stable@vger.kernel.org
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>