Blackfin: Annotate strnlen_user and strlen_user 'src' parameter with __user

The 'src' parameter of strnlen_user and strlen_user is supposed to take a
userspace pointer, so annotate it with __user. This fixes the following and
similar sparse warnings:

	fs/binfmt_elf_fdpic.c:671:36: warning: incorrect type in argument 1 (different address spaces)
	fs/binfmt_elf_fdpic.c:671:36:    expected char const *src
	fs/binfmt_elf_fdpic.c:671:36:    got char [noderef] <asn:1>*[assigned] p
	fs/binfmt_elf_fdpic.c:683:36: warning: incorrect type in argument 1 (different address spaces)
	fs/binfmt_elf_fdpic.c:683:36:    expected char const *src
	fs/binfmt_elf_fdpic.c:683:36:    got char [noderef] <asn:1>*[assigned] p

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