btrfs: add RCU locks around block group initialization

btrfs: add RCU locks around block group initialization

[ Upstream commit 29566c9c773456467933ee22bbca1c2b72a3506c ]

The space_info list is normally RCU protected and should be traversed
with rcu_read_lock held. There's a warning

  [29.104756] WARNING: suspicious RCU usage
  [29.105046] 5.6.0-rc4-next-20200305 #1 Not tainted
  [29.105231] -----------------------------
  [29.105401] fs/btrfs/block-group.c:2011 RCU-list traversed in non-reader section!!

pointing out that the locking is missing in btrfs_read_block_groups.
However this is not necessary as the list traversal happens at mount
time when there's no other thread potentially accessing the list.

To fix the warning and for consistency let's add the RCU lock/unlock,
the code won't be affected much as it's doing some lightweight
operations.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Madhuparna Bhowmik <madhuparnabhowmik10@gmail.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 7650522178477a5eef85199a73689b5c3409a783)