Btrfs: don't call free_extent_buffer twice in iterate_irefs

Avoid calling free_extent_buffer more than once when the iterator function
returns non-zero. The only code that uses this is scrub repair for corrupted
nodatasum blocks.

Signed-off-by: Jan Schmidt <list.btrfs@jan-o-sch.net>

diff --cc fs/btrfs/backref.c
Simple merge