CVE-2026-64253: kernel/fork: clear PF_BLOCK_TS in copy_process()

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

kernel/fork: clear PFBLOCKTS in copyprocess()

PFBLOCKTS is only set in blktimegetns() when current->plug is non-NULL, and blkfinishplug() clears it via blkflushplug() before NULLing the plug pointer. copyprocess() breaks the invariant by inheriting PFBLOCKTS from the parent while resetting the child's plug to NULL.

Clear PFBLOCKTS alongside that assignment so callers can rely on "PFBLOCKTS set implies current->plug != NULL" and dereference current->plug unguarded.