lib/string.c: simplify stricmp()

Removes 32 bytes on core2 with gcc 4.4.1:
   text    data     bss     dec     hex filename
   3196       0       0    3196     c7c lib/string-BEFORE.o
   3164       0       0    3164     c5c lib/string-AFTER.o

Signed-off-by: André Goddard Rosa <andre.goddard@gmail.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>