CVE-2023-53183

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

btrfs: exit gracefully if reloc roots don't match

[BUG] Syzbot reported a crash that an ASSERT() got triggered inside preparetomerge().

[CAUSE] The root cause of the triggered ASSERT() is we can have a race between quota tree creation and relocation.

This leads us to create a duplicated quota tree in the btrfsreadfsroot() path, and since it's treated as fs tree, it would have ROOTSHAREABLE flag, causing us to create a reloc tree for it.

The bug itself is fixed by a dedicated patch for it, but this already taught us the ASSERT() is not something straightforward for developers.

[ENHANCEMENT] Instead of using an ASSERT(), let's handle it gracefully and output extra info about the mismatch reloc roots to help debug.

Also with the above ASSERT() removed, we can trigger ASSERT(0)s inside mergerelocroots() later. Also replace those ASSERT(0)s with WARN_ON()s.