BACKPORT: f2fs: compress: remove unneeded preallocation

commit 8f1d49832636 upstream.

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.

Bug: 237904436
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
Change-Id: I673af616581a9e7bf711e60d1588856f24f65dc9