ovl: generalize ovl_create_workdir()

Pass in the subdir name to create and specify if subdir is persistent
or if it should be cleaned up on every mount.

Move fallback to readonly mount on failure to create dir and print of error
message into the helper.

This function is going to be used for creating the persistent 'index' dir
under workbasedir.

Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>