scsi: core: avoid preallocating big SGL for data

scsi: core: avoid preallocating big SGL for data

scsi_mq_setup_tags() preallocates a big buffer for the IO SGL. The size is
based on scsi_mq_sgl_size() which is determined based on
shost->sg_tablesize and SG_CHUNK_SIZE.

Modern DMA engines are often capable of dealing with very big segments so
the resulting scsi_mq_sgl_size() is often too big. SG_CHUNK_SIZE results in
a static 4KB SGL allocation per command.

If an HBA has lots of deep queues, preallocation for the sg list can
consume substantial amounts of memory. For lpfc, nr_hw_queues can be 70
and each queue's depth 3781. This means the resulting preallocation for
the data SGL is 70*3781*2K = 517MB.

Switch to runtime allocation for SGL for lists longer than 2 entries. This
is the approach used by NVMe PCI so it should be reasonable for SCSI as
well. Runtime SGL allocation has always been the case for the legacy I/O
path so this is nothing new.

[mkp: attempted to clarify commit desc]

Cc: Christoph Hellwig <hch@lst.de>
Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Ewan D. Milne <emilne@redhat.com>
Cc: Hannes Reinecke <hare@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>