lib/tdb: fix transaction issue for HAVE_INCOHERENT_MMAP.

lib/tdb: fix transaction issue for HAVE_INCOHERENT_MMAP.

We unmap the tdb on expand, the remap.  But when we have INCOHERENT_MMAP
(ie. OpenBSD) and we're inside a transaction, doing the expand can mean
we need to read from the database to partially fill a transaction block.
This fails, because if mmap is incoherent we never allow accessing the
database via read/write.

The solution is not to unmap and remap until we've actually written the
padding at the end of the file.

Reported-by: Amitay Isaacs <amitay@gmail.com>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Autobuild-User: Rusty Russell <rusty@rustcorp.com.au>
Autobuild-Date: Fri Mar 23 02:53:15 CET 2012 on sn-devel-104