powerpc/kuap: Avoid unnecessary reads of MD_AP

A disassembly of interrupt_exit_kernel_prepare() shows a useless read
of MD_AP register. This is shown by r9 being re-used immediately without
doing anything with the value read.

  c000e0e0:       60 00 00 00     nop
  c000e0e4: ===>  7d 3a c2 a6     mfmd_ap r9	<====
  c000e0e8:       7d 20 00 a6     mfmsr   r9
  c000e0ec:       7c 51 13 a6     mtspr   81,r2
  c000e0f0:       81 3f 00 84     lwz     r9,132(r31)
  c000e0f4:       71 29 80 00     andi.   r9,r9,32768

kuap_get_and_assert_locked() is paired with kuap_kernel_restore()
and are only used in interrupt_exit_kernel_prepare(). The value
returned by kuap_get_and_assert_locked() is only used by
kuap_kernel_restore().

On 8xx, kuap_kernel_restore() doesn't use the value read by
kuap_get_and_assert_locked() so modify kuap_get_and_assert_locked()
to not perform the read of MD_AP and return 0 instead.

The same applies on BOOKE.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/bcbc84c2dd90bb1021da792b1968cdc22112dad8.1689091022.git.christophe.leroy@csgroup.eu