Blackfin: Annotate strncpy_from_user src parameter with __user

The src parameter of strncpy_from_user is supposed to take a string from
userspace, so it should be annotated with __user. Doing so fixes the following
and similar warnings from sparse:

	kernel/sys.c:491:51: warning: incorrect type in argument 2 (different address spaces)
	kernel/sys.c:491:51:    expected char const *src
	kernel/sys.c:491:51:    got void [noderef] <asn:1>*arg
	kernel/sys.c:2061:54: warning: incorrect type in argument 2 (different address spaces)
	kernel/sys.c:2061:54:    expected char const *src
	kernel/sys.c:2061:54:    got char [noderef] <asn:1>*<noident>

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Bob Liu <lliubbo@gmail.com>