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

2 files changed, 10 insertions, 4 deletions

diff --git a/bin/df/df.1 b/bin/df/df.1
index d542dbde8067..64e471fedc42 100644
--- a/bin/df/df.1
+++ b/bin/df/df.1
@@ -29,7 +29,7 @@
 .\"     @(#)df.1	8.3 (Berkeley) 5/8/95
 .\" $FreeBSD$
 .\"
-.Dd October 5, 2020
+.Dd March 11, 2022
 .Dt DF 1
 .Os
 .Sh NAME
@@ -109,6 +109,9 @@ In conjunction with the
 or
 .Fl H
 options, the number of inodes is scaled by powers of 1000.
+In case the filesystem has no inodes then
+.Sq -
+is displayed instead of the usage percentage.
 .It Fl k
 Use 1024 byte (1 Kibibyte) blocks rather than the default.
 This overrides the
diff --git a/bin/df/df.c b/bin/df/df.c
index 63c37de92d62..627d8b1c861d 100644
--- a/bin/df/df.c
+++ b/bin/df/df.c
@@ -561,9 +561,12 @@ prtstat(struct statfs *sfsp, struct maxwidths *mwp)
 			xo_emit(format, mwp->iused, (intmax_t)used,
 			    mwp->ifree, (intmax_t)sfsp->f_ffree);
 		}
-		xo_emit(" {:inodes-used-percent/%4.0f}{U:%%} ",
-			inodes == 0 ? 100.0 :
-			(double)used / (double)inodes * 100.0);
+		if (inodes == 0)
+			xo_emit(" {:inodes-used-percent/    -}{U:} ");
+		else {
+			xo_emit(" {:inodes-used-percent/%4.0f}{U:%%} ",
+				(double)used / (double)inodes * 100.0);
+		}
 	} else
 		xo_emit("  ");
 	if (strncmp(sfsp->f_mntfromname, "total", MNAMELEN) != 0)