xfs_copy: don't use cached buffer reads until after libxfs_mount

xfs_copy: don't use cached buffer reads until after libxfs_mount

I accidentally tried to xfs_copy an ext4 filesystem, but instead of
rejecting the filesystem, the program instead crashed.  I figured out
that zeroing the superblock was enough to trigger this:

# dd if=/dev/zero of=/dev/sda bs=1024k count=1
# xfs_copy  /dev/sda /dev/sdb
Floating point exception

The exact crash happens in this line from libxfs_getbuf_flags, which is
called from the main() routine of xfs_copy:

	if (btp == btp->bt_mount->m_ddev_targp) {
		(*bpp)->b_pag = xfs_perag_get(btp->bt_mount,
				xfs_daddr_to_agno(btp->bt_mount, blkno));

The problem here is that the uncached read filled the incore superblock
with zeroes, which means mbuf.sb_agblocks is zero.  This causes a
division by zero in xfs_daddr_to_agno, thereby crashing the program.

In commit f8b581d6, we made it so that xfs_buf structures contain a
passive reference to the associated perag structure.  That commit
assumes that no program would try a cached buffer read until the buffer
cache is fully set up, which is true throughout xfsprogs... except for
the beginning of xfs_copy.  For whatever reason, it attempts an uncached
read of the superblock to figure out the real superblock size, then
performs a *cached* read with the proper buffer length and verifier.
The cached read crashes the program.

Fix the problem by changing the (second) cached read into an uncached read.

Fixes: f8b581d6 ("libxfs: actually make buffers track the per-ag structures")
Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>