powerpc/sstep: Convert to scoped user access

powerpc/sstep: Convert to scoped user access

Commit 861574d51bbd ("powerpc/uaccess: Implement masked user access")
provides optimised user access by avoiding the cost of access_ok().

Convert single step emulation functions to scoped user access to
benefit from masked user access.

Scoped user access also make the code simpler.

Signed-off-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/8f2d85bddacff18046096dc255fd94f6a0f8b230.1773137010.git.chleroy@kernel.org