When replaying a log tree with qgroups enabled, tree_mod_log_rewind does a
sanity-check of the number of items against the maximum possible number.
It calculates that number with the nodesize of fs_root. Unfortunately
fs_root is not yet set at this stage. So instead use the nodesize from
tree_root, which is already initialized.
Signed-off-by: NArne Jansen <sensille@gmx.net>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>