Kill indirect include of file.h from eventfd.h, use fdget() in cgroup.c

kernel/cgroup.c is the only place in the tree that relies on eventfd.h
pulling file.h; move that include there.  Switch from eventfd_fget()/fput()
to fdget()/fdput(), while we are at it - eventfd_ctx_fileget() will fail
on non-eventfd descriptors just fine, no need to do that check twice...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/eventfd.h

@@ -9,7 +9,6 @@
 #define _LINUX_EVENTFD_H
 
 #include <linux/fcntl.h>
-#include <linux/file.h>
 #include <linux/wait.h>
 
 /*
@@ -26,6 +25,8 @@
 #define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
 #define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
 
+struct file;
+
 #ifdef CONFIG_EVENTFD
 
 struct file *eventfd_file_create(unsigned int count, int flags);