Btrfs: fix return value for __tree_mod_log_oldest_root

In __tree_mod_log_oldest_root() we must return the found operation even if
it's not a ROOT_REPLACE operation. Otherwise, the caller assumes that there
are no operations to be rewinded and returns immediately.

The code in the caller is modified to improve readability.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>