cifs: rename cifs_iget to cifs_root_iget

The current cifs_iget isn't suitable for anything but the root inode.
Rename it with a more appropriate name.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Steve French <sfrench@us.ibm.com>