powerpc: Allow create_branch() to return errors

Currently create_branch() creates a branch instruction for you, and
patches it into the call site.  In some circumstances it would be nice
to be able to create the instruction and patch it later, and also some
code might want to check for errors in the branch creation before
doing the patching.  A future commit will change create_branch() to
check for errors.

For callers that don't care, replace create_branch() with
patch_branch(), which just creates the branch and patches it directly.

While we're touching all the callers, change to using unsigned int *,
as this seems to match usage better.  That allows (and requires) us to
remove the volatile in the definition of vector in powermac/smp.c and
mpc86xx_smp.c, that's correct because now that we're passing vector as
an unsigned int * the compiler knows that it's value might change
across the patch_branch() call.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Acked-by: Kumar Gala <galak@kernel.crashing.org>
Acked-by: Jon Loeliger <jdl@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>