btrfs: handle tree backref walk error properly

[BUG]
Smatch reports the following errors related to commit ("btrfs: output
affected files when relocation fails"):

	fs/btrfs/inode.c:283 print_data_reloc_error()
	error: uninitialized symbol 'ref_level'.

[CAUSE]
That part of code is mostly copied from scrub, but unfortunately scrub
code from the beginning is not doing the error handling properly.

The offending code looks like this:

	do {
		ret = tree_backref_for_extent();
		btrfs_warn_rl();
	} while (ret != 1);

There are several problems involved:

- No error handling
  If that tree_backref_for_extent() failed, we would output the same
  error again and again, never really exit as it requires ret == 1 to
  exit.

- Always do one extra output
  As tree_backref_for_extent() only return > 0 if there is no more
  backref item.
  This means after the last item we hit, we would output an invalid
  error message for ret > 0 case.

[FIX]
Fix the old code by:

- Move @ref_root and @ref_level into the if branch
  And do not initialize them, so we can catch such uninitialized values
  just like what we do in the inode.c

- Explicitly check the return value of tree_backref_for_extent()
  And handle ret < 0 and ret > 0 cases properly.

- No more do {} while () loop
  Instead go while (true) {} loop since we will handle @ret manually.

Reported-by: Dan Carpenter <dan.carpenter@linaro.org>
Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>