bug#70561: ln -sfr stats the wrong file

[David Leonard <d+gnubugs@adaptive-enterprises.com>]

(resend)

When analyzing file accesses with strace, I noticed that ln stats an
unrelated file. This only happens with the opions -sfr and when the dest
symlink already exists.

Here's a reproduction script:

     # setup
     mkdir -p a b
     > b/x
     ln -fs anything a/x

     # ln -sfr b/x a/x
     strace -o trace.out --decode-fds=path -e %file \
 	src/ln -sfr b/x a/x

     test $(readlink a/x) = ../b/x
     grep stat trace.out

The invalid stat that happens is:

     newfstatat(AT_FDCWD</tmp/lnbug>, "../b/x", 0x7ffdb94a0270, 0) = -1 ENOENT
(No such file or directory)

The cause seems to be in src/ln.c's do_link() where the `source` "b/x" is
converted to "../b/x" by

         if (relative)
               source = rel_source = convert_abs_rel (source, dest);

and then the test to see if deleting the destination file would delete
the source works by checking hard links on the source.

 	if (source_status != 0)
 	    source_status = stat (source, &source_stats);

Except that -r made source relative to the dest directory.