[RFC] configfs_unregister_group() API

[Al Viro <viro@zeniv.linux.org.uk>]

	AFAICS, it (and configfs_unregister_default_group())
will break if called with group non-empty (i.e. when rmdir(2)
would've failed with -ENOTEMPTY); configfs_detach_prep()
is called, but return value is completely ignored.

	Similar breakage happens in configfs_unregister_subsystem(),
but there it looks like the drivers are responsible for not calling
it that way.  It yells if configfs_detach_prep() fails and AFAICS
all callers do guarantee it never happens.

	configfs_unregister_group() is quiet; from my reading of
the callers, only pci-endpoint might end up calling it for group
that is not guaranteed to be empty.  I'm not familiar with
pci-endpoint guts, so I might very well be missing something there.

Questions to configfs API maintainers (that'd be Christoph, these
days, AFAIK)

1) should such a call be considered a driver bug?
2) should configfs_unregister_group() at least warn when that happens?

and, to pci-endpoint maintainer

3) what, if anything, prevents such calls in pci-endpoint?  Because
as it is, configfs will break badly when that happens...