ext4: fix leaking uninitialized memory in fast-commit journal

ext4: fix leaking uninitialized memory in fast-commit journal

When space at the end of fast-commit journal blocks is unused, make sure
to zero it out so that uninitialized memory is not leaked to disk.

Fixes: aa75f4d3daae ("ext4: main fast-commit commit path")
Cc: <stable@vger.kernel.org> # v5.10+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20221106224841.279231-4-ebiggers@kernel.org
Signed-off-by: Theodore Ts'o <tytso@mit.edu>