ZIL: Remove 128K into 2x68K LWB split optimization

[amotin]

To improve 128KB block write performance in case of multiple VDEVs ZIL used to spit those writes into two 68KB ones. Unfortunately it was found to cause LWB buffer overflow, trying to write maximum- sizes 128KB TX_CLONE_RANGE record with 1022 block pointers into 68KB buffer, since unlike TX_WRITE ZIL code can't split it.

This is a minimally-invasive temporary block cloning fix until the following more invasive prediction code refactoring.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[ixhamza]

Without this patch, the following test will ASSERT/Panic:

zpool destroy testpool
rm -rf /var/tmp/disk-bclone
mkdir -p /var/tmp/disk-bclone
truncate -s 2G /var/tmp/disk-bclone/a /var/tmp/disk-bclone/b /var/tmp/disk-bclone/c /var/tmp/disk-bclone/e /var/tmp/disk-bclone/f
zpool create -o feature@block_cloning=enabled testpool /var/tmp/disk-bclone/a /var/tmp/disk-bclone/b /var/tmp/disk-bclone/c log mirror /var/tmp/disk-bclone/e /var/tmp/disk-bclone/f
zfs create -o recordsize=32K testpool/testfs
dd if=/dev/urandom of=/testpool/testfs/file1 bs=32K count=1022 conv=fsync
zpool sync testpool
cp --reflink /testpool/testfs/file1 /testpool/testfs/file2
sync

I will include it in ZTS.