#2633 gather: local FS metris calculated inconsitently

[Tyrel Datwyler]

The local filesystem plugin calls statfs to collect metrics. AvailableSpace is calculated using the f_bavail field from the statfs struct whereas AvailableSpacePercentage is calculated based off of the f_bfree field. These metrics should be consistent in which field they base their calculation.

              long    f_bfree;    /* free blocks in fs */
              long    f_bavail;   /* free blocks avail to non-superuser */

Ideally, the plugin should be extended to report the free space available to non-superuser vs superuser, but that is left to be dealt with by a future feature.

[Tyrel Datwyler]

Patch attached

[Tyrel Datwyler]

Pushed to git tree

[3835ad]