ovl: whiteout inode sharing

Share inode with different whiteout files for saving inode and speeding up
delete operation.

If EMLINK is encountered when linking a shared whiteout, create a new one.
In case of any other error, disable sharing for this super block.

Note: ofs->whiteout is protected by inode lock on workdir.

Signed-off-by: Chengguang Xu <cgxu519@mykernel.net>
Reviewed-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>