ovl: check lowerdir amount for non-upper mount

Recently multi-lower layer mount support allow upperdir and workdir
to be omitted, then cause overlayfs can be mount with only one
lowerdir directory. This action make no sense and have potential risk.

This patch check the total number of lower directories to prevent
mounting overlayfs with only one directory.

Also, an error message is added to indicate lower directories exceed
OVL_MAX_STACK limit.

Signed-off-by: hujianyang <hujianyang@huawei.com>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>