[S390] Convert extmem spin_lock into a mutex.

There's no need to have a spin_lock here, but need sleepable context
for vmem_map. Therefore convert the spin_lock into a mutex.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>