fdisk: don't ignore 1MiB granularity on 512-byte sector devices

fdisk: don't ignore 1MiB granularity on 512-byte sector devices

Note that +10MB is 100000 (10^N) and relative values (+<size>
convention) should be be aligned to the default grain (= 1 MiB).

Old version:

  Last sector, +sectors or +size{K,M,G} (2048-2047999, default 2047999): +10MB
  Partition 1 of type Linux and of size 9.5 MiB is set
  ...
  /dev/loop0p1            2048       21578        9765+  83  Linux
  /dev/loop0p2           21579       43062       10742   83  Linux

New version:
  Last sector, +sectors or +size{K,M,G} (2048-2047999, default 2047999): +10MB
  Partition 1 of type Linux and of size 10 MiB is set
  ...
  /dev/loop0p1            2048       22527       10240   83  Linux
  /dev/loop0p2           22528       43007       10240   83  Linux

Addresses: http://thread.gmane.org/gmane.linux.kernel.device-mapper.dm-crypt/5913/focus=5929
Reported-by: Milan Broz <mbroz@redhat.com>
Signed-off-by: Karel Zak <kzak@redhat.com>