md: minor cleanup in safe_delay_store.

There isn't really much room for races with ->safemode_delay.
But as I am trying to clean up any racy code and will soon
be removing reconfig_mutex protection from most _store()
functions:
 - only set mddev->safemode_delay once, to ensure no code
   can see an intermediate value
 - use safemode_timer to call md_safemode_timeout() rather than
   calling it directly, to ensure it never races with itself.

Signed-off-by: NeilBrown <neilb@suse.de>