tdb: Truncate the file after expand failure

tdb: Truncate the file after expand failure

Without this it's very easy to create virtually huge files: ftruncate expands a
file, the pwrites fail with ENOSPC, thus the write fails. The next writer runs
into the same situation, and ftruncate-expands the file even further. tdb_check
will then spend ages reading the 4GB of zeros byte by byte.

Here we hold the freelist lock or are inside a transaction, so it is safe to
cut the file again. Nobody can have used the space that we have tried to
allocate, so we can't have any stray pointers corrupting the database.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>