blockdev: allow for larger values for start sector

blockdev: allow for larger values for start sector

commit 9147d2ad8a ("blockdev: Don't fail on missing start sector") limits
the size of the start sector to 10 digits.

Multi-terrabyte devices can have partitions with a start sector larger than
10 digits, which will cause an sprintf() to abort due to overflowing the buffer.

It causes:
  # blockdev --report /dev/sda4
  RO    RA   SSZ   BSZ   StartSec            Size   Device
  *** buffer overflow detected ***: terminated
  Aborted (core dumped)