bpf: Update a comment

This comment was introduced by fix [1], later the fix was refined by
change [2], and the context of the usage of m_get2() and m_getjcl()
got lost, then the comment became obscure.

Update to reflect the current behavior.

1. f13da24715a7 net/bpf: Fix writing of buffer bigger than PAGESIZE
2. a051ca72e281 Introduce m_get3()

Fixes: a051ca72e281 Introduce m_get3()
MFC after: 3 days