check_disk increase alert precision (#1989)

* check_disk increase alert precision

Free disk percentage value was rounded to a full integer meaning it alerted about ~1% percent too early. This is about 10GB on a 1TB disk.
The warning and critical thresholds already support float values, so just the percentage calculation needs to be improved.

old:

    ./check_disk -w 35% -c 20% -p / -f -vvv
    Thresholds(pct) for / warn: 35.000000 crit 20.000000
    calling stat on /
    For /, used_pct=65 free_pct=35 used_units=286451 free_units=156651 total_units=443102 used_inodes_pct=11 free_inodes_pct=89 fsp.fsu_blocksize=4096 mult=1048576
    Freespace_units result=0
    Freespace% result=1
    Usedspace_units result=0
    Usedspace_percent result=0
    Usedinodes_percent result=0
    Freeinodes_percent result=0
    DISK WARNING - free space: WARNING [ / 156651MiB (35% inode=89%)];| /=300365643776B;302006979788;371700898201;0;464626122752

new:

    ./check_disk -w 35% -c 20% -p / -f -vvv
    Thresholds(pct) for / warn: 35.000000 crit 20.000000
    calling stat on /
    For /, used_pct=64.649722 free_pct=35.350278 used_units=286464 free_units=156637 total_units=443102 used_inodes_pct=10.016183 free_inodes_pct=89.983817 fsp.fsu_blocksize=4096 mult=1048576
    Freespace_units result=0
    Freespace% result=0
    Usedspace_units result=0
    Usedspace_percent result=0
    Usedinodes_percent result=0
    Freeinodes_percent result=0
    DISK OK - free space: / 156637MiB (35.4% inode=90%);| /=300379275264B;302006979788;371700898201;0;464626122752

* check_disk: adjust test case to support float precision

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