generic: test fsync of directory with renamed symlink

generic: test fsync of directory with renamed symlink

Test that if we fsync a directory, create a symlink inside it, rename
the symlink, fsync again the directory and then power fail, after the
filesystem is mounted again, the symlink exists with the new name and
it has the correct content.

This currently fails on btrfs, because the symlink ends up empty (which
is illegal on Linux), but it is fixed by kernel commit:

    d0e64a981fd841 ("btrfs: always log symlinks in full mode")

Reviewed-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Zorro Lang <zlang@kernel.org>