nvmf: Rescan namespaces after reconnecting

[jhb]

Summary

In particular, check to see if namespaces have been added or removed
while disconnected.

Sponsored by: Chelsio Communications

[imp]

Is this a good place to add a brief note about why this is a good idea, either in the commit message or in the code? (If so, I'd lean towards the former).

[jhb]

In D45461#1036800, @imp wrote:

Is this a good place to add a brief note about why this is a good idea, either in the commit message or in the code? (If so, I'd lean towards the former).

Oh, yeah, I probably should note that a namespace could have been removed or added while we were disconnected in the log message. That's the use case, though after the change to optionally fail I/O's while disconnected it is also needed to "see" existing namespaces after a reconnect. For this commit though in the time sequence it is just the "something changed while disconnected" (which is also how I tested it, using ctladm to add/remove LUNs on the server side while the client was disconnected).