fs/eventfd.c: make eventfd_ida static

Fix sparse warning:

fs/eventfd.c:26:1: warning:
 symbol 'eventfd_ida' was not declared. Should it be static?

Link: http://lkml.kernel.org/r/20190413142348.34716-1-yuehaibing@huawei.com
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>