Some features can only work when all layers are on the same fs.  Test this
condition during mount time, so features can check them later.

Add helper ovl_same_sb() to return the common super block in case all
layers are on the same fs.
Signed-off-by: NAmir Goldstein <amir73il@gmail.com>
Signed-off-by: NMiklos Szeredi <mszeredi@redhat.com>