tdb2: suppress tdb1 backend logging when locking returns EINTR or EAGAIN

tdb2: suppress tdb1 backend logging when locking returns EINTR or EAGAIN

The TDB1 code logs multiple times on errors; we must prevent that in
the limited case where locking fails.  With TDB2, this can happen due
to the lock function attribute, where the user supplies replacement
locking functions which are allowed to return with errno EAGAIN or
EINTR for various special-effects.  Flooding the logs for this is
unfriendly.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
(Imported from CCAN commit a391b2b900bc6d5c0467869a454bdb5c51b5a3be)