xfs_copy: simplify first_agbno calculation

xfs_copy: simplify first_agbno calculation

After ffe9a9a xfsprogs: xfs_copy: fix data corruption of target,
xfs_copy started hitting an ASSERT for a 4k sector / 4k blocksize
filesystem:

# dd if=/dev/zero of=test.img bs=1M count=1024
# mkfs.xfs -s size=4096 test.img
# xfs_copy test.img xfs.img
xfs_copy: xfs_copy.c:720: main: Assertion `((((((xfs_daddr_t)(3 << (mp)->m_sectbb_log)) + 1) * (1<<9)) + first_residue) % source_blocksize) == 0' failed.
Aborted

I started digging through all the calculations below, and realized
that in the end, all it wants is the first filesystem block after
the AG header.  XFS_AGFL_BLOCK(mp) + 1 suffices for this purpose;
rip out the rest which seems overly complex and apparently bug-prone.

I tested this by creating a 4g filesystem with combinations of
sector & block size between 512 and 4k, copying in /lib/modules,
running an xfs_copy of that, and running repair against the copy;
it all looks good.  It took a long time, but I will create a
simpler/shorter xfstest based on this.

Reported-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>