uprobes: simplify find_active_uprobe_rcu() VMA checks

At the point where find_active_uprobe_rcu() is used we know that VMA in
question has triggered software breakpoint, so we don't need to validate
vma->vm_flags. Keep only vma->vm_file NULL check.

Suggested-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Link: https://lkml.kernel.org/r/20241122035922.3321100-2-andrii@kernel.org