btrfs: use the new bit BTRFS_FS_TREE_MOD_LOG_USERS at btrfs_free_tree_block()

Instead of exposing implementation details of the tree mod log to check
if there are active tree mod log users at btrfs_free_tree_block(), use
the new bit BTRFS_FS_TREE_MOD_LOG_USERS for fs_info->flags instead. This
way extent-tree.c does not need to known about any of the internals of
the tree mod log and avoids taking a lock unnecessarily as well.

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>