btrfs: ref-verify: free ref cache before clearing mount opt

As clearing REF_VERIFY mount option indicates there were some errors in a
ref-verify process, a ref cache is not relevant anymore and should be
freed.

btrfs_free_ref_cache() requires REF_VERIFY option being set so call
it just before clearing the mount option.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.

Reported-by:  <syzbot+be14ed7728594dc8bd42@syzkaller.appspotmail.com>
Fixes: fd708b81 ("Btrfs: add a extent ref verify tool")
CC: stable@vger.kernel.org # 5.4+
Closes: https://lore.kernel.org/lkml/000000000000e5a65c05ee832054@google.com/


Reported-by:  <syzbot+c563a3c79927971f950f@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/lkml/0000000000007fe09705fdc6086c@google.com/


Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Fedor Pchelkin <pchelkin@ispras.ru>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>