mkfs_minix: use get_volume_size_in_bytes instead of local version

mkfs_minix: use get_volume_size_in_bytes instead of local version

Hopefully this also closes 4730

function                                             old     new   delta
valid_offset                                          55       -     -55
mkfs_minix_main                                     2925    2674    -251
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(add/remove: 0/1 grow/shrink: 0/1 up/down: 0/-306)           Total: -306 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>