x86/math-emu: Rename frstor()

This is in the way of renaming the low level hardware accessors to match
the instruction name. Prepend it with FPU_ which is consistent vs. the
rest of the emulation code.

No functional change.

  [ bp: Correct the Reported-by: ]

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121454.111665161@linutronix.de