powerpc/ptrace: Convert gpr32_set_common_user() to scoped user access

powerpc/ptrace: Convert gpr32_set_common_user() to scoped user access

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

Convert gpr32_set_common_user() to scoped user access to benefit
from masked user access.

Scoped user access also make the code simpler.

Also changes label from Efault to efault to avoid checkpatch
complaining about CamelCase.

Signed-off-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/2409643daab08b4bc07004c2b88f42085d1ef45a.1773136838.git.chleroy@kernel.org