tee: shm: Remove refcounting of kernel pages

tee: shm: Remove refcounting of kernel pages

Earlier TEE subsystem assumed to refcount all the memory pages to be
shared with TEE implementation to be refcounted. However, the slab
allocations within the kernel don't allow refcounting kernel pages.

It is rather better to trust the kernel clients to not free pages while
being shared with TEE implementation. Hence, remove refcounting of kernel
pages from register_shm_helper() API.

Fixes: b9c0e49abfca ("mm: decline to manipulate the refcount on a slab page")
Reported-by: Marco Felsch <m.felsch@pengutronix.de>
Reported-by: Sven Püschel <s.pueschel@pengutronix.de>
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Co-developed-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Signed-off-by: Sumit Garg <sumit.garg@oss.qualcomm.com>
Tested-by: Sven Püschel <s.pueschel@pengutronix.de>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>