6.12 backport 'Fix for "fragnesia" (CVE-2026-46300) and variants'

Salvatore Bonaccorsorequested to merge
carnil/linux:6.12-fragnesia into debian/6.12/trixie

This backports merge request !1950 (merged)

Commits:

  • net: skbuff: propagate shared-frag marker through frag-transfer helpers
  • net: skbuff: preserve shared-frag marker during coalescing (CVE-2026-46300)

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