tdb2: cleanup oob handling.

tdb2: cleanup oob handling.

The tdb_oob() function can fail due to errors, as well as because the length
asked for is greater than the size of the file.  Clean that up:

(1) If probe is true, only fail if there's an error, not if the length is
    too great.
(2) Exit tdb_open() if it tdb_oob() probe fails; this helps cut down
    test time for failtest.
(3) Don't set probe to true in tdb_direct() fail; a minor issue, but it means
    we log failure.