powerpc/32s: Convert switch_mmu_context() to C

switch_mmu_context() does things that can easily be done in C.

For updating user segments, we have update_user_segments().

As mentionned in commit b5efec00 ("powerpc/32s: Move KUEP
locking/unlocking in C"), update_user_segments() has the loop
unrolled which is a significant performance gain.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/05c0875ad8220c03452c3a334946e207c6ca04d6.1622708530.git.christophe.leroy@csgroup.eu