x86, fpu: Change __thread_fpu_begin() to use use_eager_fpu()

__thread_fpu_begin() checks X86_FEATURE_EAGER_FPU by hand, we have
a helper for that.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Link: http://lkml.kernel.org/r/20140902175720.GA21656@redhat.com


Reviewed-by: Suresh Siddha <sbsiddha@gmail.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>