powerpc/code-patching: Don't call is_vmalloc_or_module_addr() without CONFIG_MODULES

If CONFIG_MODULES is not set, there is no point in checking
whether text is in module area.

This reduced the time needed to activate/deactivate ftrace
by more than 10% on an 8xx.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/f3c701cce00a38620788c0fc43ff0b611a268c54.1647962456.git.christophe.leroy@csgroup.eu