nvme: export controller reconnect event count via sysfs

nvme: export controller reconnect event count via sysfs

When an NVMe-oF link goes down, the driver attempts to recover the
connection by repeatedly reconnecting to the remote controller at
configured intervals. A maximum number of reconnect attempts is also
configured, after which recovery stops and the controller is removed
if the connection cannot be re-established.

The driver maintains a counter, nr_reconnects, which is incremented on
each reconnect attempt. However if in case the reconnect is successful
then this counter reset to zero. Moreover, currently, this counter is
only reported via kernel log messages and is not exposed to userspace.
Since dmesg is a circular buffer, this information may be lost over
time.

So introduce a new accumulator which accumulates nr_reconnect attempts
and also expose this accumulator per-fabric ctrl via a new sysfs
attribute reconnect_count, under diag attribute grroup to provide
persistent visibility into the number of reconnect attempts made by the
host. This information can help users diagnose unstable links or
connectivity issues. Furthermore, this sysfs attribute is also writable
so user may reset it to zero, if needed.

The reconnect_count can also be consumed by monitoring tools such as
nvme-top to improve controller-level observability.

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>