powerpc64/modules: correctly iterate over stubs in setup_ftrace_ool_stubs

CONFIG_PPC_FTRACE_OUT_OF_LINE introduced setup_ftrace_ool_stubs() to
extend the ppc64le module .stubs section with an array of
ftrace_ool_stub structures for each patchable function.

Fix its ppc64_stub_entry stub reservation loop to properly write across
all of the num_stubs used and not just the first entry.

Fixes: eec37961 ("powerpc64/ftrace: Move ftrace sequence out of line")
Signed-off-by: Joe Lawrence <joe.lawrence@redhat.com>
Acked-by: Naveen N Rao (AMD) <naveen@kernel.org>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250912142740.3581368-3-joe.lawrence@redhat.com