kernel: bump 6.12 to 6.12.42

kernel: bump 6.12 to 6.12.42

Changelog: https://cdn.kernel.org/pub/linux/kernel/v6.x/ChangeLog-6.12.42

Added backport to fix ipv6 breakage with the 6.12.42 release:
generic/backport-6.12/621-proc-fix-missing-pde_set_flags.patch[1]

All other patches automatically rebased.

1. https://lore.kernel.org/all/20250821105806.1453833[email protected]

Build system: x86/64
Build-tested: flogic/xiaomi_redmi-router-ax6000-ubootmod, ramips/tplink_archer-a6-v3, x86/64-glibc
Run-tested: flogic/xiaomi_redmi-router-ax6000-ubootmod, ramips/tplink_archer-a6-v3, x86/64-glibc (Intel N150)

Signed-off-by: John Audia <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/19618
Signed-off-by: Robert Marko <[email protected]>