Intended for passing temporary free space in GConv.
int sp_fhfilehandle to opended file or device
int sp_file:1flag if sp_fh is handle file (else to device)
block_t sp_firstfirst usable block numbered from 0, for files always 0
block_t sp_lengthnumber of usable blocks on this file/device, for files its maximal length
block_t sp_indexinternal for GConv, first block of this free space in global linear addressing in GConv