handle unlimited backfill limit (backfillLimit = -1) for GsiClient in scans

Query supports unlimited backfill.

The maximum size of temporary backfill files (MB). Setting the size to 0 disables backfill. Setting the size to -1 means the size is unlimited.

From GsiClient, we don't backfill if the value is <= 0; we should handle the same.

config - `query_tmpspace_limit`

we can set the value to be math.MaxInt64 if the value is < 0 else we can also use current disk capacity as the limit