MFC r332120:

MFC r332120:
  If a user closes the socket before we call tcp_usr_abort(), then
  tcp_drop() may unlock the INP.  Currently, tcp_usr_abort() does not
  check for this case, which results in a panic while trying to unlock
  the already-unlocked INP (not to mention, a use-after-free violation).

  Make tcp_usr_abort() check the return value of tcp_drop(). In the case
  where tcp_drop() returns NULL, tcp_usr_abort() can skip further steps
  to abort the connection and simply unlock the INP_INFO lock prior to
  returning.

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