docs: it_IT: fix namespace collisions at locking.rst

The C domain functions there collide with the English ones,
due to namespace collision, generating lots of warnings with
Sphinx 3.x:

	./include/linux/mutex.h:121: WARNING: Duplicate C declaration, also defined in 'translations/it_IT/kernel-hacking/locking'.
	Declaration is 'mutex_init'.
	./include/linux/mutex.h:152: WARNING: Duplicate C declaration, also defined in 'translations/it_IT/kernel-hacking/locking'.
	Declaration is 'mutex_is_locked'.
	./include/linux/mutex.h:226: WARNING: Duplicate C declaration, also defined in 'translations/it_IT/kernel-hacking/locking'.
	Declaration is 'mutex_trylock_recursive'.
	./kernel/locking/mutex.c:281: WARNING: Duplicate C declaration, also defined in 'translations/it_IT/kernel-hacking/locking'.
	Declaration is 'mutex_lock'.
	...

Add a namespace tag there, in order to prevent that.

Acked-by: Federico Vaga <federico.vaga@vaga.pv.it>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>