common/rc: add _min()

Add a helper to find the minimum of two numbers.

A similar helper is being added in xfstests:
https://lore.kernel.org/linux-xfs/[email protected]/T/#m962683d8115979e57342d2644660230ee978c803

Signed-off-by: John Garry <[email protected]>
Signed-off-by: Shin'ichiro Kawasaki <[email protected]>

Author: johnpgarry

common/rc

@@ -701,3 +701,14 @@ _real_dev()
 	fi
 	echo "$dev"
 }
+
+_min() {
+	local ret
+
+	for arg in "$@"; do
+		if [ -z "$ret" ] || (( arg < ret )); then
+			ret="$arg"
+		fi
+	done
+	echo "$ret"
+}