bpf, docs: Use consistent names for the same field

Use consistent names for the same field, e.g., 'dst' vs 'dst_reg'.
Previously a mix of terms were used for the same thing in various cases.

Signed-off-by: Dave Thaler <dthaler@microsoft.com>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/r/20230127224555.916-1-dthaler1968@googlemail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>