MFC r286139:

MFC r286139:

  Do not allocate the buffers at opening of the descriptor, because once
  the buffer is allocated we are committed to a particular buffer method
  (BPF_BUFMODE_BUFFER in this case).

  If we are using zero-copy buffers, the userland program must register its
  buffers before set the interface.

  If we are using kernel memory buffers, we can allocate the buffer at the
  time that the interface is being set.

  This fix allows the usage of BIOCSETBUFMODE after r235746.

  Update the comments to reflect the recent changes.

  Sponsored by: Rubicon Communications (Netgate)

git-svn-id: svn://svn.freebsd.org/base/stable/10@286850 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f