f2fs: fix incorrect free inode count in ->statfs

While calculating inode count that we can create at most in the left space,
we should consider space which data/node blocks occupied, since we create
data/node mixly in main area. So fix the wrong calculation in ->statfs.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>