x86/fpu: Use pkru_write_default() in copy_init_fpstate_to_fpregs()

There is no point in using copy_init_pkru_to_fpregs() which in turn calls
write_pkru(). write_pkru() tries to fiddle with the task's xstate buffer
for nothing because the XRSTOR[S](init_fpstate) just cleared the xfeature
flag in the xstate header which makes get_xsave_addr() fail.

It's a useless exercise anyway because the reinitialization activates the
FPU so before the task's xstate buffer can be used again a XRSTOR[S] must
happen which in turn dumps the PKRU value.

Get rid of the now unused copy_init_pkru_to_fpregs().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121455.732508792@linutronix.de