38.321 6.1.3.1 says: "The size of the RLC headers and MAC subheaders are
not considered in the buffer size computation" [for BSR]. However, prior
to this commit, we take the size signaled of the BSR and schedule the UE
with that amount of data, making it impossible for the UE to transmit
all its data at once (because it is the size signaled + headers).

In this commit, over-estimate the buffer status size. As we cannot know
the exact number and size of headers, we simply use "BSR size + n", as
done in function overestim_bsr_index().