powerpc: Make create_branch() return errors if the branch target is too large

If you pass a target value to create_branch() which is more than 32MB - 4,
or - 32MB away from the branch site, then it's impossible to create an
immediate branch.  The current code doesn't check, which will lead to us
creating a branch to somewhere else - which is bad.

For code that cares to check we return 0, which is easy to check for, and
for code that doesn't at least we'll be creating an illegal instruction,
rather than a branch to some random address.

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