xfs_copy: Fix meta UUID handling on multiple copies

xfs_copy: Fix meta UUID handling on multiple copies

Zorro reported that when making multiple copies of a V5
filesystem with xfs_copy while generating new UUIDs, all
but the first copy were corrupt.

Upon inspection, the corruption was related to incorrect UUIDs;
the original UUID, as stamped into every metadata structure,
was not preserved in the sb_meta_uuid field of the superblock
on any but the first copy.

This happened because sb_update_uuid was using the UUID present in
the ag_hdr structure as the unchanging meta-uuid which is to match
existing structures, but it also /updates/ that UUID with the
new identifying UUID present in tcarg.  So the newly-generated
UUIDs moved transitively from tcarg->uuid to ag_hdr->xfs_sb->sb_uuid
to ag_hdr->xfs_sb->sb_meta_uuid each time the function got called.

Fix this by looking instead to the unchanging, original UUID
present in the xfs_sb_t we are given, which reflects the original
filesystem's metadata UUID, and copy /that/ UUID into each target
filesystem's meta_uuid field.

Most of this patch is changing comments and re-ordering tests
to match; the functional change is to simply use the *sb rather
than the *ag_hdr to identify the proper metadata UUID.

Reported-and-tested-by: Zorro Lang <zlang@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>