Currently a negative dentry is cached whenever a non-existing file was
looked up over TFTP. Short of a barebox reset, there is no way to
invalidate that dentry, so barebox retries the look up.

Fix this by always reporting TFTP dentries as invalid in the d_revalidate
callback.

Signed-off-by: Ahmad Fatoum <ahmad@a3f.at>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>