mount: remove directory if mounting fails

mount: remove directory if mounting fails

Aside from just cleaning up mountd's tmp dir it's also important for
autofs support. With such an empty directory autofs_process_request
believed device is mounted and it kept replying AUTOFS_IOC_READY to the
kernel. It was causing a log storm like:
mountd[1015]: Got a autofs packet
mountd[1015]: kernel is requesting a mount -> sda1
mountd[1015]: Got a autofs packet
mountd[1015]: kernel is requesting a mount -> sda1
mountd[1015]: Got a autofs packet
mountd[1015]: kernel is requesting a mount -> sda1

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>