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

Vulnerability Description

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.

References

• https://git.kernel.org/stable/c/c847b28a799733b04574060ab9d00f215970627d
• https://git.kernel.org/stable/c/71291aa7246645ef622621934d2067400380645e
• https://git.kernel.org/stable/c/c60676b81fab456b672796830f6d8057058f029c
• https://git.kernel.org/stable/c/728d4d045b628e006b48a448f3326a7194c88d32
• https://git.kernel.org/stable/c/9cc887ac24b7a0598f4042ae9af6b9a33072f75b
• https://git.kernel.org/stable/c/7d1df13bf078ffebfedd361d714ff6cee1ff01b9
• https://git.kernel.org/stable/c/03804641ec2d0da4fa088ad21c88e703d151ce16
• https://git.kernel.org/stable/c/b8ccef048354074a548f108e51d0557d6adfd3a3