x86/fpu/regset: Move fpu__read_begin() into regset

The function can only be used from the regset get() callbacks safely. So
there is no reason to have it globally exposed.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121453.234942936@linutronix.de