ata: libata-scsi: scale DSM TRIM payload by MAX PAGES PER DSM COMMAND

DSM TRIM currently always emits a single 512-byte page of LBA Range Entries
(ATA_MAX_TRIM_RNUM == 64 ranges), regardless of how many pages the device
can accept in one DATA SET MANAGEMENT command.

The maximum is reported by MAX PAGES PER DSM COMMAND (IDENTIFY DEVICE word
105).  Honour it: size the TRIM descriptor as a whole number of 512-byte
pages, bounded by that limit and by the logical sector size (the WRITE SAME
data-out buffer is a single logical block).  Build and transfer only as
many pages as the request needs, and set the DSM COUNT field, qc->nbytes
and the maximum WRITE SAME length in the Block Limits VPD page accordingly.

Build the descriptor straight into the WRITE SAME data-out buffer using an
atomic sg_miter mapping, instead of staging it in the shared ata_scsi_rbuf
and copying it out.  This removes the global ata_scsi_rbuf_lock and a
memcpy from the TRIM path.

While commit 9379e6b8e0f9 ("libata: Safely overwrite attached page in WRITE
SAME xlat") replaced direct access to the data-out buffer with an
intermediate step that writes the entries in the ata_scsi_rbuf buffer, this
solution writes to the data-out buffer using sg_miter, which maps each
segment with kmap_atomic (SG_MITER_ATOMIC), so it's highmem- and
multi-segment-safe, and it's usable from the non-sleeping
command-submission path (unlike the page_address() access that
ata_scsi_rbuf originally replaced).

A 512-byte-sector device still uses a single page, so its behaviour is
unchanged.

Add ata_id_dsm_max_pages() to read IDENTIFY DEVICE word 105.

Reviewed-by: Hannes Reinecke <hare@kernel.org>
Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>

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