bpf: Rename nfct_bsa to nfct_btf_struct_access

The former name was a little hard to guess.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/73adc72385c8b162391fbfb404f0b6d4c5cc55d7.1663683114.git.dxu@dxuuu.xyz


Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>