fs: Set the size of empty dirs to 0.

Before the make_empty_dir_inode calls were introduce into proc, sysfs,
and sysctl those directories when stated reported an i_size of 0.
make_empty_dir_inode started reporting an i_size of 2.  At least one
userspace application depended on stat returning i_size of 0.  So
modify make_empty_dir_inode to cause an i_size of 0 to be reported for
these directories.

Cc: stable@vger.kernel.org
Reported-by: Tejun Heo <tj@kernel.org>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>