block/aoe: discover_timer: Convert timers to use timer_setup()

In preparation for unconditionally passing the struct timer_list pointer to
all timer callbacks, switch to using the new timer_setup() and from_timer()
to pass the timer pointer explicitly.

This refactors the discover_timer to remove the needless locking and
state machine used for synchronizing timer death. Using del_timer_sync()
will already do the right thing.

Cc: Jens Axboe <axboe@kernel.dk>
Cc: "Ed L. Cashin" <ed.cashin@acm.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Kees Cook <keescook@chromium.org>