btrfs: qgroup: use atomic operations for btrfs_fs_info::qgroup_flags

btrfs: qgroup: use atomic operations for btrfs_fs_info::qgroup_flags

Currently we define btrfs_fs_info::qgroup_flags as u64, to match the
on-disk qgroup status item's flag.

But for now we have only 4 bits utilized for that flag, and since it's
u64 we have no way to properly use the existing atomic bit operations
(requires an unsigned long pointer).

This results in a lot of non-atomic operations inside qgroup code. Some
maybe fine as other locks are involved, but still it's not a good
practice.

Remove those non-atomic operations by:

- Re-define btrfs_fs_info::qgroup_flags as unsigned long
- Define BTRFS_QGROUP_STATUS_BIT_* and BTRFS_QGROUP_RUNTIME_BIT_*
  Instead of the old value define the bit number.

- Use set_bit()/clear_bit()/test_bit() to replace open-coded bit
  operations

- Add one extra check at qgroup status item read time
  To make sure the on-disk flag is still inside ULONG_MAX.
  Otherwise reject the status item and disable qgroup.

- Get rid of unnecessary spinlock when checking a single bit

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>