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