btrfs: use BTRFS_FS_UPDATE_UUID_TREE_GEN flag for UUID tree rescan check

The UUID tree rescan check in open_ctree() compares
fs_info->generation with the superblock's uuid_tree_generation.
This comparison is not reliable because fs_info->generation is
bumped at transaction start time in join_transaction(), while
uuid_tree_generation is only updated at commit time via
update_super_roots().

Between the early BTRFS_FS_UPDATE_UUID_TREE_GEN flag check and the
late rescan decision, mount operations such as file orphan cleanup
from an unclean shutdown start transactions without committing
them. This advances fs_info->generation past uuid_tree_generation
and produces a false-positive mismatch.

Use the BTRFS_FS_UPDATE_UUID_TREE_GEN flag directly instead. The
flag was already set earlier in open_ctree() when the generations
were known to match, and accurately represents "UUID tree is up to
date" without being affected by subsequent transaction starts.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Dave Chen <davechen@synology.com>
Signed-off-by: Robbie Ko <robbieko@synology.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>