ntdb: make sure file is always a multiple of PAGESIZE (now NTDB_PGSIZE)

ntdb: make sure file is always a multiple of PAGESIZE (now NTDB_PGSIZE)

ntdb uses tdb's transaction code, and it has an undocumented but implicit
assumption: that the transaction recovery area is always aligned to the
transaction pagesize.  This means that no block will overlap the recovery
area.

This is maintained by rounding the size of the database up, so do the same
for ntdb.

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