lib/string.c: simplify strnstr()

Signed-off-by: André Goddard Rosa <andre.goddard@gmail.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Joe Perches <joe@perches.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>