usage:
echo <pid/used/free> trace > /proc/memdump
echo used > /proc/memdump //output all used memory info with backtrace
echo free > /proc/memdump //output all free memory info
echo 22 > /proc/memdump //output used memory info for task pid is 22 with backtrace
Signed-off-by: Jiuzhu Dong <dongjiuzhu1@xiaomi.com>