UPSTREAM: drm/shmem-helper: Avoid vm_open error paths

commit 09bf649a upstream.

vm_open() is not allowed to fail.  Fortunately we are guaranteed that
the pages are already pinned, thanks to the initial mmap which is now
being cloned into a forked process, and only need to increment the
refcnt.  So just increment it directly.  Previously if a signal was
delivered at the wrong time to the forking process, the
mutex_lock_interruptible() could fail resulting in the pages_use_count
not being incremented.

Bug: 260569666
Fixes: 2194a63a ("drm: Add library for shmem backed GEM objects")
Cc: stable@vger.kernel.org
Signed-off-by: Rob Clark <robdclark@chromium.org>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20221130185748.357410-3-robdclark@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I1f6d481c677e0ee4747352f7949570bccf39815c
Signed-off-by: Lee Jones <joneslee@google.com>