eventfd: strictly check the count parameter of eventfd_write to avoid inputting illegal strings

Since eventfd's document has clearly stated: A write(2) call adds
the 8-byte integer value supplied in its buffer to the counter.

However, in the current implementation, the following code snippet
did not cause an error:

char str[16] = "hello world";
uint64_t value;
ssize_t size;
int fd;

fd = eventfd(0, 0);
size = write(fd, &str, strlen(str));
printf("eventfd: test writing a string, size=%ld\n", size);
size = read(fd, &value, sizeof(value));
printf("eventfd: test reading as uint64, size=%ld, valus=0x%lX\n",
size, value);

close(fd);

And its output is:
eventfd: test writing a string, size=8
eventfd: test reading as uint64, size=8, valus=0x6F77206F6C6C6568

By checking whether count is equal to sizeof(ucnt), such errors
could be detected. It also follows the requirements of the manual.

Signed-off-by: Wen Yang <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Cc: Alexander Viro <[e-mail]>
Cc: Jens Axboe <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Jan Kara <[e-mail]>
Cc: David Woodhouse <[e-mail]>
Cc: Matthew Wilcox <[e-mail]>
Cc: Eric Biggers <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Signed-off-by: Christian Brauner <[e-mail]>

fs/eventfd.c

···
 	ssize_t res;
 	__u64 ucnt;
 
-	if (count < sizeof(ucnt))
+	if (count != sizeof(ucnt))
 		return -EINVAL;
 	if (copy_from_user(&ucnt, buf, sizeof(ucnt)))
 		return -EFAULT;