xfs_scrub: handle media scans of internal rt devices correctly

xfs_scrub: handle media scans of internal rt devices correctly

Codex noticed media scans of internal rt devices don't work at all
correctly.  First, we fail to allocate a ctx->verify_disks[XFS_DEV_RT]
for the internal rt section, and even if we did, phase 6 doesn't
allocate media_verify_state.rvp[XFS_DEV_RT] if there's a media error on
an internal rt volume, so we'll crash there too.

Fix both issues to make it work properly.

Cc: linux-xfs@vger.kernel.org # v6.15.0
Fixes: 37591ef3f4f14c ("xfs_scrub: support internal RT device")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>