x86/fpu: Always allow FPU in interrupt if use_eager_fpu()

The __thread_has_fpu() check in interrupted_kernel_fpu_idle() was needed
to prevent the nested kernel_fpu_begin(). Now that we have in_kernel_fpu
and !__thread_has_fpu() case in __kernel_fpu_begin() does not depend on
use_eager_fpu() (except clts) we can remove it.

__thread_has_fpu() can be false even if use_eager_fpu(), but this case
does not differ from !use_eager_fpu() case except we should not worry
about X86_CR0_TS, __kernel_fpu_begin()/end() will not touch this bit.

Note: I think we can kill all irq_fpu_usable() checks except in_kernel_fpu,
just we need to record the state of X86_CR0_TS in __kernel_fpu_begin() and
conditionalize stts() in __kernel_fpu_end(), but this needs another patch.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Acked-by: Andy Lutomirski <luto@amacapital.net>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Suresh Siddha <sbsiddha@gmail.com>
Link: http://lkml.kernel.org/r/20150119185151.GC16427@redhat.com
Signed-off-by: Borislav Petkov <bp@suse.de>

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