ovl: factor out ovl_free_entry() and ovl_stack_*() helpers

In preparation for moving lowerstack into ovl_inode.

Note that in ovl_lookup() the temp stack dentry refs are now cloned
into the final ovl_lowerstack instead of being transferred, so cleanup
always needs to call ovl_stack_free(stack).

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