fs/partition/msdos: fix unusable extended partition for > 512B sector

Smaller size than a minimum blocksize can't be used, after all it's
handled like 0 size.

For extended partition itself, this makes sure to use bigger size than one
logical sector size at least.

Signed-off-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Cc: Daniel Taylor <Daniel.Taylor@wdc.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>