[S390] dasd calls kzalloc while holding a spinlock.

The dasd function dasd_set_uid calls kzalloc while holding the
dasd_devmap_lock. Rearrange the code to do the memory allocation
outside the lock.

Signed-off-by: Horst Hummel <horst.hummel@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>