fix buffer overflow while reading /proc/cmdline

char line[255] was used to store each line in vold.fstab. but, by AutoVolume changes, it is also (ab)used to store each arg in /proc/cmdline.
length of /proc/cmdline is architecture dependent. it should be 1024 on arm. so each arg can be longer than 255.

this change fixes vold crash (stack corruption detected: aborted) with longer arg.

this issue is found by ^BatVanko^ on forum.xda-developers.com. thank you!

Change-Id: I7a4a9dd00a9a0b291429df9a85fbe097fd22c1ba