mv-common.h: increase malloc arena to 4MiB

This will fix the following error:

---8<---
UBIFS error (pid 0): ubifs_mount: Error reading superblock on volume 'ubi:root' errno=-12!
--->8---

Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>
Cc: prafulla@marvell.com
Cc: dimax.main@gmail.com
Tested-by: Alex Xol <dimax.main@gmail.com>