ntdb: reduce race between creating file and getting open lock.

ntdb: reduce race between creating file and getting open lock.

In tdb, we grab the open lock immediately after we open the file.  In
ntdb, we usually did some work first.  tdbtorture managed to get in
before the creator grabbed the lock:

	testing with 3 processes, 5000 loops, seed=1338246020
	ntdb:torture.ntdb:IO Error:ntdb_open: torture.ntdb is not a ntdb file
	29023:torture.ntdb:db open failed

At cost of a little duplicated code, we can reduce the race.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>