libxfs: remove the volname concept

libxfs: remove the volname concept

IRIX has the concept of a volume that has data/log/rt subvolumes (that's
where the subvolume name in Linux comes from), but in the current
Linux-only xfsprogs version trying to pretend we do anything with that
it is just utterly confusing.  The volname is basically just a very
obsfucated second way to pass the data device name, so get rid of it
in the libxfs and progs internals.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>
Signed-off-by: Carlos Maiolino <cem@kernel.org>