ext4: fix off-by-one errors in fast-commit block filling

In the Linux kernel, the following vulnerability has been resolved:

ext4: fix off-by-one errors in fast-commit block filling

Due to several different off-by-one errors, or perhaps due to a late
change in design that wasn't fully reflected in the code that was
actually merged, there are several very strange constraints on how
fast-commit blocks are filled with tlv entries:

- tlvs must start at least 10 bytes before the end of the block, even
though the minimum tlv length is 8. Otherwise, the replay code will
ignore them. (BUG: ext4_fc_reserve_space() could violate this
requirement if called with a len of blocksize - 9 or blocksize - 8.
Fortunately, this doesn't seem to happen currently.)

- tlvs must end at least 1 byte before the end of the block. Otherwise
the replay code will consider them to be invalid. This quirk
contributed to a bug (fixed by an earlier commit) where uninitialized
memory was being leaked to disk in the last byte of blocks.

Also, strangely these constraints don't apply to the replay code in
e2fsprogs, which will accept any tlvs in the blocks (with no bounds
checks at all, but that is a separate issue...).

Given that this all seems to be a bug, let's fix it by just filling
blocks with tlv entries in the natural way.

Note that old kernels will be unable to replay fast-commit journals
created by kernels that have this commit.