Btrfs: add missing read locks in backref.c

iref_to_path and iterate_irefs both increment the eb's refcount to use it
after releasing the path. Both depend on consistent data remaining in the
extent buffer and need a read lock to protect it.

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

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