eventfd: change int to __u64 in eventfd_signal()

eventfd_ctx->count is an __u64 counter which is allowed to reach
ULLONG_MAX.  eventfd_write() adds a __u64 value to "count", but the kernel
side eventfd_signal() only adds an int value to it.  Make them consistent.

[akpm@linux-foundation.org: update interface documentation]
Signed-off-by: Sha Zhengju <handai.szj@taobao.com>
Cc: Davide Libenzi <davidel@xmailserver.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>