busybox: fix busybox lock applet pidstr buffer overflow

busybox: fix busybox lock applet pidstr buffer overflow

Kernel setting `/proc/sys/kernel/pid_max` can be set up to 4194304 (7
digits) which will cause buffer overflow in busbox lock patch, this
often happens when running in a rootfs container environment.
This commit enlarges `pidstr` to 12 bytes to ensure a sufficient buffer
for pid number and an additional char '\n'.

Signed-off-by: Qichao Zhang <njuzhangqichao@gmail.com>