md: setup safemode_timer before it's being used

We used to set up the safemode_timer timer in md_run. If md_run
would fail before the timer was set up we'd end up trying to modify
a timer that doesn't have a callback function when we access safe_delay_store,
which would trigger a BUG.

neilb: delete init_timer() call as setup_timer() does that.

Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Signed-off-by: NeilBrown <neilb@suse.com>