df: do not report a 100% full inode usage on fs without inodes

Before:
devfs 2 2 0 100% 0 0 100% /dev

After:
devfs 2 2 0 100% 0 0 - /dev

The previous behaviour was confusing for end users and many monitoring tools

Note the linux df tools is also using the same syntax '-' for such filesystem

MFC After: 2 weeks
Reviewed by: manu, emaste, imp
Differential Revision: https://reviews.freebsd.org/D34515