fs: limit filesystem stacking depth

Add a simple read-only counter to super_block that indicates deep this
is in the stack of filesystems.  Previously ecryptfs was the only
stackable filesystem and it explicitly disallowed multiple layers of
itself.

Overlayfs, however, can be stacked recursively and also may be stacked
on top of ecryptfs or vice versa.

To limit the kernel stack usage we must limit the depth of the
filesystem stack.  Initially the limit is set to 2.

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