md/md-llbitmap: raise barrier before state machine transition

In the Linux kernel, the following vulnerability has been resolved:

md/md-llbitmap: raise barrier before state machine transition

Move the barrier raise operation before calling llbitmap_state_machine()

in both llbitmap_start_write() and llbitmap_start_discard(). This

ensures the barrier is in place before any state transitions occur,

preventing potential race conditions where the state machine could

complete before the barrier is properly raised.