kern/fs: Allow number of blocks in block list to be optional, defaulting length to...

kern/fs: Allow number of blocks in block list to be optional, defaulting length to device length

This is primarily useful to do something like "loopback newdev (dev)8+" to
create a device that skips the first 4 KiB, which may contain a container
header, e.g. a non-standard RAID1 header, that GRUB does not recognize. This
would allow that container data to be potentially accessed up to the end of
container, which may be necessary for some layouts that store data at the
end. There is currently not a good way to programmatically get the number
of sectors on a disk to set the appropriate length of the blocklist.

Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>