srcu: Add whitespace to __SRCU_STRUCT_INIT() & __DEFINE_SRCU()

This is a whitespace-only commit with no change in functionality.
Its purpose is to prepare for later commits that: (1) Cause statically
allocated srcu_struct structures to rely on compile-time initialization
and (2) Move fields from the srcu_struct structure to a new srcu_usage
structure.

Cc: Christoph Hellwig <hch@lst.de>
Tested-by: Sachin Sant <sachinp@linux.ibm.com>
Tested-by: "Zhang, Qiang1" <qiang1.zhang@intel.com>
Tested-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>