powerpc/64s: Blacklist rtas entry/exit from kprobes

We can't take traps with relocation off, so blacklist enter_rtas() and
rtas_return_loc(). However, instead of blacklisting all of enter_rtas(),
introduce a new symbol __enter_rtas from where on we can't take a trap
and blacklist that.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>