n_gsm: add missing __user annotations

sparse warns about incorrect types:
n_gsm.c:2638:35: warning: incorrect type in argument 1 (different address spaces)
n_gsm.c:2638:35:    expected void [noderef] <asn:1> *to
n_gsm.c:2638:35:    got void *

The ioctl handler casts its `arg' to (void *) without __user. Add that.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20200219084949.28074-5-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>