exec: Don't use cpu_index to detect if cpu_exec_init()'s been called

exec: Don't use cpu_index to detect if cpu_exec_init()'s been called

Instead use QTAIL's tqe_prev field to detect if cpu's been
placed in list by cpu_exec_init() which is always set if
QTAIL element is in list.

Fixes SIGSEGV on failure path in case cpu_index is assigned
by board and cpu.relalize() fails before cpu_exec_init() is called.

In follow up patches, cpu_index will be assigned by boards that
support cpu hot(un)plug and need stable cpu_index that doesn't
depend on order cpus are created/removed.

Signed-off-by: Igor Mammedov <[email protected]>
Reported-by: David Gibson <[email protected]>
Reviewed-by: David Gibson <[email protected]>
Signed-off-by: Eduardo Habkost <[email protected]>