padata: Avoid nested calls to cpus_read_lock() in pcrypt_init_padata()

pcrypt_init_padata()
   cpus_read_lock()
   padata_alloc_possible()
     padata_alloc()
       cpus_read_lock()

The nested call to cpus_read_lock() works with the current implementation,
but prevents the conversion to a percpu rwsem.

The other caller of padata_alloc_possible() is pcrypt_init_padata() which
calls from a cpus_read_lock() protected region as well.

Remove the cpus_read_lock() call in padata_alloc() and document the
calling convention.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-crypto@vger.kernel.org
Link: http://lkml.kernel.org/r/20170524081547.571278910@linutronix.de