ZTS: Make bc conditional use compatible with new BSD bc

FreeBSD recently replaced the GNU bc and dc in the base system with
BSD licensed versions.  They are supposed to be compatible with all
the features present in the GNU versions, but it turns out they are
picky about `if` statements having a corresponding `else`.  ZTS uses
`echo "if ($x > $y) 1" | bc` in a few places, which causes tests to
fail unexpectedly with the new bc.

Change the two expressions in ZTS to `if ($x > $y) 1 else 0` for
compatibility with the new BSD bc.

Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ryan Moeller <ryan@iXsystems.com>
Closes #10551

tests/zfs-tests/include/math.shlib

@@ -30,14 +30,15 @@ function within_percent
 	typeset percent=$3
 
 	# Set $a or $b to $2 such that a >= b
-	[[ '1' = $(echo "if ($2 > $a) 1" | bc) ]] && a=$2 || b=$2
+	[[ '1' = $(echo "if ($2 > $a) 1 else 0" | bc) ]] && a=$2 || b=$2
 
 	# Prevent division by 0
 	[[ $a =~ [1-9] ]] || return 1
 
 	typeset p=$(echo "scale=2; $b * 100 / $a" | bc)
 	log_note "Comparing $a and $b given $percent% (calculated: $p%)"
-	[[ '1' = $(echo "scale=2; if ($p >= $percent) 1" | bc) ]] && return 0
+	[[ '1' = $(echo "scale=2; if ($p >= $percent) 1 else 0" | bc) ]] && \
+	    return 0
 
 	return 1
 }