Manage: fix check after dereference issue

Manage: fix check after dereference issue

The code dereferences dev_st earlier without checking, it gives SAST
problem.

dev_st is needed for attempt_re_add(), but it is executed only if
dv->disposition != 'S', so move disposition check up.

tst is a must to reach this place, dup_super() have to return valid
pointer, all it needs to check is if load_super() returns superblock.

Signed-off-by: Mariusz Tkaczyk <mariusz.tkaczyk@linux.intel.com>