f2fs: compress: remove unneeded preallocation

We will reserve iblocks for compression saved, so during compressed
cluster overwrite, we don't need to preallocate blocks for later
write.

In addition, it adds a bug_on to detect wrong reserved iblock number
in __f2fs_cluster_blocks().

Bug fix in the original patch by Jaegeuk:
If we released compressed blocks having an immutable bit, we can see less
number of compressed block addresses. Let's fix wrong BUG_ON.

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