btrfs: clean up our handling of refs == 0 in snapshot delete

In the Linux kernel, the following vulnerability has been resolved:

btrfs: clean up our handling of refs == 0 in snapshot delete

In reada we BUG_ON(refs == 0), which could be unkind since we aren't

holding a lock on the extent leaf and thus could get a transient

incorrect answer. In walk_down_proc we also BUG_ON(refs == 0), which

could happen if we have extent tree corruption. Change that to return

-EUCLEAN. In do_walk_down() we catch this case and handle it correctly,

however we return -EIO, which -EUCLEAN is a more appropriate error code.

Finally in walk_up_proc we have the same BUG_ON(refs == 0), so convert

that to proper error handling. Also adjust the error message so we can

actually do something with the information.