uprobes: Remove redundant spinlock in uprobe_deny_signal()

Since clearing a bit in thread_info is an atomic operation, the spinlock
is redundant and can be removed, reducing lock contention is good for
performance.

Signed-off-by: Liao Chang <liaochang1@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20250124093826.2123675-2-liaochang1@huawei.com