fs/sfs: Fix over-read of root object name

fs/sfs: Fix over-read of root object name

There's a read of the name of the root object that assumes that the name
is nul-terminated within the root block. This isn't guaranteed - it seems
SFS would require you to read multiple blocks to get a full name in general,
but maybe that doesn't apply to the root object.

Either way, figure out how much space is left in the root block and don't
over-read it. This fixes some OOB reads.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>