btrfs: make btrfs_super_block::log_root_transid deprecated

When using "btrfs inspect-internal dump-super" to inspect an fs with
dirty log, it always shows the log_root_transid as 0:

  log_root                30474240
  log_root_transid        0 <<<
  log_root_level          0

It turns out that, btrfs_super_block::log_root_transid is never really
utilized (even no read for it).

This can date back to the introduction of btrfs into upstream kernel.

In fact, when reading log tree root, we always use
btrfs_super_block::generation + 1 as the expected generation.
So here we're completely safe to mark this member deprecated.

In theory we can easily reuse this member for other purposes, but to be
extra safe, here we follow the leafsize way, by adding "__unused_" for
log_root_transid.
And we can safely remove the accessors, since there is no such callers
from the very beginning.

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