btrfs: remove unnecessary mutex lock in qgroup_account_snapshot

The quota status used to be tracked as a variable, so the mutex was
needed (until "Btrfs: add a flags field to btrfs_fs_info" afcdd129).
Since the status is a bit modified atomically and we don't hold the
mutex beyond the check, we can drop it.

Signed-off-by: David Sterba <dsterba@suse.com>