This patch tries to speedup fzero_range by making space preallocation and
address removal of blocks in one dnode page as in batch operation.

In virtual machine, with zram driver:

dd if=/dev/zero of=/mnt/f2fs/file bs=1M count=4096
time xfs_io -f /mnt/f2fs/file -c "fzero 0 4096M"

Before:
real	0m3.276s
user	0m0.008s
sys	0m3.260s

After:
real	0m1.568s
user	0m0.000s
sys	0m1.564s
Signed-off-by: Chao Yu <yuchao0@huawei.com>
[Jaegeuk Kim: consider ENOSPC case]
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>