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

[bapt]

before:

$ df -i /dev
devfs                       2        2         0   100%       0          0   100%   /dev

after:

$ df -i /dev
devfs                       2        2         0   100%       0          0      -   /dev

[bapt]

it avoids monitoring tools which look for inode occupation to have to maintain of list of filesystem to skip.

Note that on linux df is doring the same, for example on msdosfs filesystem.

[manu]

make sense to me.

[emaste]

Seems reasonable to me. Perhaps we should note it in the man page?

[imp]

In D34515#782020, @emaste wrote:

Seems reasonable to me. Perhaps we should note it in the man page?

+1