riscv: Fix register corruption from uninitialized cregs on error

riscv: Fix register corruption from uninitialized cregs on error

compat_riscv_gpr_set() calls cregs_to_regs() unconditionally, even when
user_regset_copyin() fails. Since cregs is an uninitialized stack
variable, a copyin failure causes uninitialized stack data to be written
into the target task's pt_regs, corrupting its register state and
potentially leaking kernel stack contents.

compat_restore_sigcontext() has the same issue: it calls cregs_to_regs()
even when __copy_from_user() fails, leading to the same corruption of
the signal-returning task's register state on error.

Only call cregs_to_regs() when the user copy succeeds.

Fixes: 4608c159594f ("riscv: compat: ptrace: Add compat_arch_ptrace implement")
Fixes: 7383ee05314b ("riscv: compat: signal: Add rt_frame implementation")
Signed-off-by: Michael Neuling <mikey@neuling.org>
Assisted-by: Cursor:claude-4.6-opus-high-thinking
Link: https://patch.msgid.link/20260501062320.2339562-1-mikey@neuling.org
Signed-off-by: Paul Walmsley <pjw@kernel.org>