inet: fix potential deadlock in reqsk_queue_unlink()

commit 83fccfc3 upstream.

When replacing del_timer() with del_timer_sync(), I introduced
a deadlock condition :

reqsk_queue_unlink() is called from inet_csk_reqsk_queue_drop()

inet_csk_reqsk_queue_drop() can be called from many contexts,
one being the timer handler itself (reqsk_timer_handler()).

In this case, del_timer_sync() loops forever.

Simple fix is to test if timer is pending.

Fixes: 2235f2ac ("inet: fix races with reqsk timers")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Cc: Holger Hoffstätte <holger.hoffstaette@googlemail.com>
Cc: Andre Tomt <andre@tomt.net>
Cc: Chris Caputo <ccaputo@alt.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>