Fix dbufstats_001_pos

Implement a new helper within_tolerance to test if a value
is within range of a target.

Because the dbufstats and dbufs kstat file are being read
at slightly different times, it is possible for stats to be
slightly off. Use within_tolerance to determine if the value
is "close enough" to the target.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #7239 
Closes #7266

tests/zfs-tests/include/math.shlib

@@ -42,6 +42,29 @@ function within_percent
 	return 1
 }
 
+#
+# Return 0 if value is within +/-tolerance of target.
+# Return 1 if value exceeds our tolerance.
+# For use like this:
+#
+# Do $action if value is within the tolerance from target passed in:
+#	within_tolerance VAL TAR TOL && $action
+# Do $action if value surpasses the tolerance from target passed in:
+#	within_tolerance VAL TAR TOL || $action
+#
+function within_tolerance #value #target #tolerance
+{
+	typeset val=$1
+	typeset target=$2
+	typeset tol=$3
+
+	typeset diff=$((abs(val - target)))
+	log_note "Checking if $val is within +/-$tol of $target (diff: $diff)"
+	((diff <= tol)) && return 0
+
+	return 1
+}
+
 #
 # Return 0 if the human readable string of the form <value>[suffix] can
 # be converted to bytes.  Allow suffixes are shown in the table below.