Allocate one FMR pool per SRP connection instead of one SRP pool
per HCA. This improves scalability of the SRP initiator.

Only request the SCSI mid-layer to retry a SCSI command after a
temporary mapping failure (-ENOMEM) but not after a permanent
mapping failure. This avoids that SCSI commands are retried
indefinitely if a permanent memory mapping failure occurs.

Tell the SCSI mid-layer to reduce queue depth temporarily in the
unlikely case where an application is queuing many requests with
more than max_pages_per_fmr sg-list elements.

For FMR pool allocation, base the max_pages_per_fmr parameter on
the HCA memory registration limit. Only try to allocate an FMR
pool if FMR is supported.
Signed-off-by: NBart Van Assche <bvanassche@acm.org>
Signed-off-by: NRoland Dreier <roland@purestorage.com>