arm64: poe: fix stale POR_EL0 values for ptrace

arm64: poe: fix stale POR_EL0 values for ptrace

If a process wrote to POR_EL0 and then crashed before a context switch
happened, the coredump would contain an incorrect value for POR_EL0.

The value read in poe_get() would be a stale value left in thread.por_el0.  Fix
this by reading the value from the system register, if the target thread is the
current thread.

This matches what gcs/fpsimd do.

Fixes: 175198199262 ("arm64/ptrace: add support for FEAT_POE")
Reported-by: David Spickett <david.spickett@arm.com>
Cc: stable@vger.kernel.org
Signed-off-by: Joey Gouly <joey.gouly@arm.com>
Cc: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Kevin Brodsky <kevin.brodsky@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>