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.
(Imported from CCAN commit
77658070a3e4f712b94d659b2e399031ce3394c8)
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>