jump_label: Make it possible for arches to invoke jump_label_init() earlier

Some arches (powerpc at least) would like to invoke jump_label_init()
much earlier in boot. So check static_key_initialized in order to make
sure this function runs only once.

LGTM-by: Ingo (http://marc.info/?l=linux-kernel&m=144049104329961&w=2

)
Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>