zfs-functions.in: is_mounted() always returns 1

The 'while read line; ...; done' loop is run in a piped subshell 
therefore the 'return 0' would not cause a return from the 
is_mounted() function.  In all cases, this function will 
always return 1.

The fix is to 'return 1' from the subshell on a successful match 
(no match == return 0), and then negating the final return value.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: TerraTech <TerraTech@users.noreply.github.com>
Closes #8151

etc/init.d/zfs-functions.in

@@ -423,9 +423,14 @@ is_mounted()
 	mount | \
 	    while read line; do
 		if echo "$line" | grep -q " on $mntpt "; then
-		    return 0
+		    # returns:
+		    #   0 on unsuccessful match
+		    #   1 on a successful match
+		    return 1
 		fi
 	    done
 
-	return 1
+	# The negation will flip the subshell return result where the default
+	# return value is 0 when a match is not found.
+	return $(( !$? ))
 }