btrfs: delete pointless BUG_ON check on quota root in btrfs_qgroup_account_extent()

[ Upstream commit f40a3ea94881f668084f68f6b9931486b1606db0 ]

The BUG_ON is deep in the qgroup code where we can expect that it
exists. A NULL pointer would cause a crash.

It was added long ago in 550d7a2e ("btrfs: qgroup: Add new qgroup
calculation function btrfs_qgroup_account_extents()."). It maybe made
sense back then as the quota enable/disable state machine was not that
robust as it is nowadays, so we can just delete it.

Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 5ae1493c5eac1a7a7ced34970a24cb3a5680a63b)
[Vegard: fix conflict in context due to missing commit
 c9f6f3cd1c6fc4df959ce2bce15e5e6ce660bfd4 ("btrfs: qgroup: Allow
 trace_btrfs_qgroup_account_extent() to record its transid").]
Signed-off-by: Vegard Nossum <vegard.nossum@oracle.com>