f2fs: increase the limit for reserve_root

This patch increases the threshold that limits the reserved root space from 0.2%
to 12.5% by using simple shift operation.

Typically Android sets 128MB, but if the storage capacity is 32GB, 0.2% which is
around 64MB becomes too small. Let's relax it.

Cc: stable@vger.kernel.org
Reported-by: Aran Dalton <arda@allwinnertech.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>