MFC r286260:

MFC r286260:
  Remove the mtx_sleep() from the kqueue f_event filter.

  The filter is called from the network hot path and must not sleep.

  The filter runs with the descriptor lock held and does not manipulate the
  buffers, so it is not necessary sleep when the hold buffer is in use.

  Just ignore the hold buffer contents when it is being copied to user space
  (when hold buffer in use is set).

  This fix the "Sleeping thread owns a non-sleepable lock" panic when the
  userland thread is too busy reading the packets from bpf(4).

  PR:           200323
  Sponsored by: Rubicon Communications (Netgate)

git-svn-id: svn://svn.freebsd.org/base/stable/10@286859 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f