powerpc/ftrace: Have arch_ftrace_get_regs() return NULL unless FL_SAVE_REGS is set

When FL_SAVE_REGS is not set we get here via ftrace_caller()
which doesn't save all registers.

ftrace_caller() explicitely clears regs.msr, so we can rely
on it to know where we come from. We don't expect MSR register
to be 0 at all when involving ftrace.

Fixes: 40b035ef

 ("powerpc/ftrace: Implement CONFIG_DYNAMIC_FTRACE_WITH_ARGS")
Reported-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/2f9a7e898c93cc7438ef5ccd47cb9c3a9c5b53ef.1644949750.git.christophe.leroy@csgroup.eu