bcachefs: Add missing synchronize_srcu_expedited() call when shutting down

We use the polling interface to srcu for tracking pending frees; when
shutting down we don't need to wait for an srcu barrier to free them,
but SRCU still gets confused if we shutdown with an outstanding grace
period.

Reported-by:  <syzbot+6a038377f0a594d7d44e@syzkaller.appspotmail.com>
Reported-by:  <syzbot+0ece6edfd05ed20e32d9@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>