fs/ext2: allow reserving zero block for root

fs/ext2: allow reserving zero block for root

The previous default, zero, just meant "use the default value of the
filesystem generator", which happened to be 5% (the traditional value
for all ext-creating tools we've ever seen).

So, change the new default accordingly to 5%.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>