KVM: arm64: Sync SPSR_EL1 when injecting an exception into a pVM

KVM: arm64: Sync SPSR_EL1 when injecting an exception into a pVM

When pKVM injects a synchronous exception into a protected guest, it
re-enters without restoring the guest's EL1 sysregs and writes the EL1
exception registers to hardware by hand: ESR_EL1 and ELR_EL1, but not
SPSR_EL1. enter_exception64() sets SPSR_EL1 (the interrupted PSTATE)
only in memory, so the guest's handler reads a stale SPSR_EL1 and
restores the wrong PSTATE on eret.

Write SPSR_EL1 alongside the other exception registers.

Fixes: 6c30bfb18d0b ("KVM: arm64: Add handlers for protected VM System Registers")
Reported-by: sashiko <sashiko@sashiko.dev>
Signed-off-by: Fuad Tabba <tabba@google.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://patch.msgid.link/20260612113414.1022901-1-tabba@google.com
Signed-off-by: Marc Zyngier <maz@kernel.org>