kernel: ovl: fix null pointer dereference in ovl_permission()

In the Linux kernel, the following vulnerability has been resolved: ovl: fix null pointer dereference in ovlpermission Following process: P1 P2 pathlookupat linkpathwalk inodepermission ovlpermission ovlipathrealinode, &realpath path-dentry = ovlidentryupperinode dropcache dentrykillovldentry...