eventfd: show the EFD_SEMAPHORE flag in fdinfo

The EFD_SEMAPHORE flag should be displayed in fdinfo,
as different value could affect the behavior of eventfd.

Suggested-by: Christian Brauner <[e-mail]>
Signed-off-by: Wen Yang <[e-mail]>
Cc: Alexander Viro <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Christoph Hellwig <[e-mail]>
Cc: Dylan Yudaken <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Eric Biggers <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Message-Id: <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

fs/eventfd.c

···
 	/*
 	 * Every time that a write(2) is performed on an eventfd, the
 	 * value of the __u64 being written is added to "count" and a
-	 * wakeup is performed on "wqh". A read(2) will return the "count"
-	 * value to userspace, and will reset "count" to zero. The kernel
-	 * side eventfd_signal() also, adds to the "count" counter and
-	 * issue a wakeup.
+	 * wakeup is performed on "wqh". If EFD_SEMAPHORE flag was not
+	 * specified, a read(2) will return the "count" value to userspace,
+	 * and will reset "count" to zero. The kernel side eventfd_signal()
+	 * also, adds to the "count" counter and issue a wakeup.
 	 */
 	__u64 count;
 	unsigned int flags;
···
 		   (unsigned long long)ctx->count);
 	spin_unlock_irq(&ctx->wqh.lock);
 	seq_printf(m, "eventfd-id: %d\n", ctx->id);
+	seq_printf(m, "eventfd-semaphore: %d\n",
+		   !!(ctx->flags & EFD_SEMAPHORE));
 }
 #endif