journal_extend — extend buffer credits.
int journal_extend (handle_t * handle, int nblocks);
Some transactions, such as large extends and truncates, can be done atomically all at once or in several stages. The operation requests a credit for a number of buffer modications in advance, but can extend its credit if it needs more.
journal_extend tries to give the running handle more buffer credits. It does not guarantee that allocation - this is a best-effort only. The calling process MUST be able to deal cleanly with a failure to extend here.
Return 0 on success, non-zero on failure.
return code < 0 implies an error return code > 0 implies normal transaction-full status.