Fix comments style (use kernel-doc style) and content to clarify some
functions. Also fix some functions signature indentation and remove a
useless blank line in sd_zbc_read_zones().

No functional change is introduced by this patch.
Signed-off-by: NDamien Le Moal <damien.lemoal@wdc.com>
Reviewed-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NMartin K. Petersen <martin.petersen@oracle.com>