libxfs: split out a libxfs_dev structure from struct libxfs_init

Most of the content of libxfs_init is members duplicated for each of the
data, log and RT devices.  Split those members into a separate
libxfs_dev structure.

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>