powerpc/modules: Use WARN_ON() in stub_for_addr()

Use WARN_ON(), while running out of stubs in stub_for_addr()
and abort loading of the module instead of BUG_ON().

Signed-off-by: Kamalesh Babulal <kamalesh@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>