powerpc/kprobes: Handle error returned by set_memory_rox()

set_memory_rox() can fail.

In case it fails, free allocated memory and return NULL.

Link: https://github.com/KSPP/linux/issues/7


Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/b4907cf4339bd086abc40430d91311436cb0c18e.1708078401.git.christophe.leroy@csgroup.eu