ext4: make sure ext4_append() always allocates new block

[ Upstream commit b8a04fe7

 ]

ext4_append() must always allocate a new block, otherwise we run the
risk of overwriting existing directory block corrupting the directory
tree in the process resulting in all manner of problems later on.

Add a sanity check to see if the logical block is already allocated and
error out if it is.

Cc: stable@kernel.org
Signed-off-by: Lukas Czerner <lczerner@redhat.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Link: https://lore.kernel.org/r/20220704142721.157985-2-lczerner@redhat.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>