read_allocated block may return block number 0, which is just an indicator
a chunk of the file is not backed by a block, i.e. it is sparse.
During file deletions, just continue with the next logical block, for other
operations treat blocknumber <= 0 as an error.
For writes, blocknumber 0 should never happen, as U-Boot always allocates
blocks for the whole file. Reading already handles this correctly, i.e. the
read buffer is 0-fillled.
Not treating block 0 as sparse block leads to FS corruption, e.g.
./sandbox/u-boot -c 'host bind 0 ./sandbox/test/fs/3GB.ext4.img ;
ext4write host 0 0 /2.5GB.file 1 '
The 2.5GB.file from the fs test is actually a sparse file.
Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
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