rcu: Remove "extern" from function declarations in include/linux/*rcu*.h

Function prototypes don't need to have the "extern" keyword since this
is the default behavior. Its explicit use is redundant.  This commit
therefore removes them.

Signed-off-by: Teodora Baluta <teobaluta@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --cc include/linux/rculist.h
Simple merge

diff --cc include/linux/rcupdate.h
Simple merge

diff --cc include/linux/rcutiny.h
Simple merge

diff --cc include/linux/rcutree.h
Simple merge