static_call: Replace pointless WARN_ON() in static_call_module_notify()

static_call_module_notify() triggers a WARN_ON(), when memory allocation
fails in __static_call_add_module().

That's not really justified, because the failure case must be correctly
handled by the well known call chain and the error code is passed
through to the initiating userspace application.

A memory allocation fail is not a fatal problem, but the WARN_ON() takes
the machine out when panic_on_warn is set.

Replace it with a pr_warn().

Fixes: 9183c3f9 ("static_call: Add inline static call infrastructure")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/8734mf7pmb.ffs@tglx