nvme: export command retry count via sysfs

When Advanced Command Retry Enable (ACRE) is configured, a controller
may interrupt command execution and return a completion status
indicating command interrupted with the DNR bit cleared. In this case,
the driver retries the command based on the Command Retry Delay (CRD)
value provided in the completion status.

Currently, these command retries are handled entirely within the NVMe
driver and are not visible to userspace. As a result, there is no
observability into retry behavior, which can be a useful diagnostic
signal.

Expose a per-namespace sysfs attribute command_retries_count, under
diag attribute group to provide visibility into retry activity. This
information can help identify controller-side congestion under load
and enables comparison across paths in multipath setups (for example,
detecting cases where one path experiences significantly more retries
than another under identical workloads).

This exported metric is intended for diagnostics and monitoring tools
such as nvme-top, and does not change command retry behavior. A new
sysfs attribute named "command_retries_count" is added for this purpose.
This attribute is both readable as well as writable. So user could
reset this counter if needed.

Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Nilay Shroff <nilay@linux.ibm.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>

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