s390/dasd: Stamp a format label into newly formatted volumes

When a CDL volume is formatted, write a small on-disk label so the format
can later be recognised by the kernel. The next patch will use this for
ESE detection.

The label records a magic, a version, whether the volume is ESE, and
whether it was formatted quick (space released, thin) or full.

It lives in track 0, head 0, record 4 (the first non-special CDL record).
R4 is written by the same channel program that formats track 0
- its WRITE_CKD transfers count + the label data instead of count-only -
so label and track format reach the disk atomically; a valid magic then
marks a completed format without a separate, racy write.

Quick vs full is derived from a full space release (RAS) preceding the
format: dasd_eckd_release_space_full() sets a per-device flag the next
format consumes. Non-ESE volumes and formats without a preceding full
release are recorded as full.

struct dasd_format_label is exactly 512 bytes (the smallest block size)
so it fits one record; larger blocks zero-pad the rest.

Reviewed-by: Jan Höppner <hoeppner@linux.ibm.com>
Signed-off-by: Stefan Haberland <sth@linux.ibm.com>
Link: https://patch.msgid.link/20260805111612.1285190-16-sth@linux.ibm.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

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