powerpc/32s: Speed up likely path of kuap_update_sr()

In most cases, kuap_update_sr() will update a single segment
register.

We know that first update will always be done, if there is no
segment register to update at all, kuap_update_sr() is not
called.

Avoid recurring calculations and tests in that case.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/848f18d213b8341939add7302dc4ef80cc7a12e3.1620307636.git.christophe.leroy@csgroup.eu