lib/strutils: parse_size() fix frac digit calculation

lib/strutils: parse_size() fix frac digit calculation

Old code:

	./test_strutils --size 0.5MiB
		0.5MiB :               512000 :     500K :      500 KiB
	./test_strutils --size 0.50MiB
		0.50MiB :              5120000 :     4.9M :      4.9 MiB

New code:
	./test_strutils --size 0.5MiB
		0.5MiB :               524288 :     512K :      512 KiB
	./test_strutils --size 0.50MiB
	       0.50MiB :               524288 :     512K :      512 KiB

Note that the new implementation also does not use float points,
because we need to support PiB and so on... it seems good enough for
things like:

        ./test_strutils --size 7.13G
                7.13G :           7656104581 :     7.1G :      7.1 GiB
        ./test_strutils --size 7.16G
                7.16G :           7690675814 :     7.2G :      7.2 GiB

to avoid situation where cfdisk creates partition with completely
crazy numbers.

Addresses: https://github.com/karelzak/util-linux/issues/782
Signed-off-by: Karel Zak <kzak@redhat.com>