[RFC/CFT] Resize support for geom_mirror

[Andrey V. Elsukov]

Hi All,

I prepared the patch, that adds new verb "resize" to the gmirror(8) and
such functionality to the geom_mirror(4).

The main idea is to be able expand the mirror size after replacing all
its components. But reducing the size also is possible.

Also I added g_resize method, and now it automatically writes gmirror's
metadata in the new last sector, when the parent provider changes the size.

Several notes about implementation:
* `gmirror resize gm0` without exactly specified size will automatically
calculate the maximum possible size for this mirror.

* Reduce the size of the mirror when it is in use is possible only with
debugflags=16.

Pawel, please look at the case, when the size of the mirror reduced when
synchronization is in progress. I guess it is enough to change the
condition in g_mirror_sync_request?

The patch is here:
	http://people.freebsd.org/~ae/g_mirror_resize.diff
-- 
WBR, Andrey V. Elsukov

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