mtd: mtdpart: ignore error -ENOENT from parsers on subpartitions

mtd: mtdpart: ignore error -ENOENT from parsers on subpartitions

Commit 5c2f7727d437 ("mtd: mtdpart: check for subpartitions parsing
result") introduced some kind of regression with parser on subpartitions
where if a parser emits an error then the entire parsing process from the
upper parser fails and partitions are deleted.

Not checking for error in subpartitions was originally intended as
special parser can emit error also in the case of the partition not
correctly init (for example a wiped partition) or special case where the
partition should be skipped due to some ENV variables externally
provided (from bootloader for example)

One example case is the TRX partition where, in the context of a wiped
partition, returns a -ENOENT as the trx_magic is not found in the
expected TRX header (as the partition is wiped)

To better handle this and still keep some kind of error tracking (for
example to catch -ENOMEM errors or -EINVAL errors), permit parser on
subpartition to emit -ENOENT error, print a debug log and skip them
accordingly.

This results in giving better tracking of the status of the parser
(instead of returning just 0, dropping any kind of signal that there is
something wrong with the parser) and to some degree restore the original
logic of the subpartitions parse.

(worth to notice that some special partition might have all the special
header present for the parser and declare 0 partition in it, this is why
it would be wrong to simply return 0 in the case of a special partition
that is NOT init for the scanning parser)

Cc: stable@vger.kernel.org
Fixes: 5c2f7727d437 ("mtd: mtdpart: check for subpartitions parsing result")
Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>