x86/fpu: Make sure x86_task_fpu() doesn't get called for PF_KTHREAD|PF_USER_WORKER tasks during exit

fpu__drop() and arch_release_task_struct() calls x86_task_fpu()
unconditionally, while the FPU context area will not be present
if it's the init task, and should not be in use when it's some
other type of kthread.

Return early for PF_KTHREAD or PF_USER_WORKER tasks. The debug
warning in x86_task_fpu() will catch any kthreads attempting to
use the FPU save area.

Fixed-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250409211127.3544993-7-mingo@kernel.org

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