pstore: Fail to unlink if a driver has not defined pstore_erase

pstore_erase is used to erase the record from the persistent store.
So if a driver has not defined pstore_erase callback return
-EPERM instead of unlinking a file as deleting the file without
erasing its record in persistent store will give a wrong impression
to customers.
Signed-off-by: Aruna Balakrishnaiah <aruna@linux.vnet.ibm.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Tony Luck <tony.luck@intel.com>

fs/pstore/inode.c

@@ -178,6 +178,8 @@ static int pstore_unlink(struct inode *dir, struct dentry *dentry)
 	if (p->psi->erase)
 		p->psi->erase(p->type, p->id, p->count,
 			      dentry->d_inode->i_ctime, p->psi);
+	else
+		return -EPERM;
 
 	return simple_unlink(dir, dentry);
 }