mkfs.mfs:add comment about the overflow check.
mkfs.mfs:add comment about the overflow check.

Change-Id: I1f09c3a62377164296037f6dc9b7874023123311

veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi,veritas-shine/minix3-rpi