powerpc/align: Convert emulate_spe() to scoped user access

powerpc/align: Convert emulate_spe() to scoped user access

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

Convert emulate_spe() 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/4ff83cb240da4e2d0c34e2bce4b8b6ef19a33777.1773136880.git.chleroy@kernel.org