bcachefs fixes for 6.15-rc5

Lots of assorted small fixes...

- Some repair path fixes, a fix for -ENOMEM when reconstructing lots of
  alloc info on large filesystems, upgrade for ancient 0.14 filesystems,
  etc.

- Various assert tweaks; assert -> ERO, ERO -> log the error in the
  superblock and continue

- casefolding now uses d_ops like on other casefolding filesystems

- fix device label create on device add, fix bucket array resize on
  filesystem resize

- fix xattrs with FORTIFY_SOURCE builds with gcc-15/clang
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