Add sanity checks for both `tlv_len` and `tlv_bitmap_len` before
decoding data from `event_buf`.

This prevents any malicious or buggy firmware from overflowing
`event_buf` through large values for `tlv_len` and `tlv_bitmap_len`.
Suggested-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/d4f8780527d551552ee96f17a0229e02e1c200d1.1692931954.git.gustavoars@kernel.org