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.
Sponsored by: Netflix, Inc.