printk: fix parsing of "brl=" option

Commit bbeddf52 ("printk: move braille console support into separate
braille.[ch] files") moved the parsing of braille-related options into
_braille_console_setup(), changing the type of variable str from char*
to char**.  In this commit, memcmp(str, "brl,", 4) was correctly updated
to memcmp(*str, "brl,", 4) but not memcmp(str, "brl=", 4).

Update the code to make "brl=" option work again and replace memcmp()
with strncmp() to make the compiler able to detect such an issue.

Fixes: bbeddf52 ("printk: move braille console support into separate braille.[ch] files")
Link: http://lkml.kernel.org/r/20160823165700.28952-1-nicolas.iooss_linux@m4x.orgSigned-off-by: Nicolas Iooss <nicolas.iooss_linux@m4x.org>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/printk/braille.c

@@ -9,10 +9,10 @@
 
 char *_braille_console_setup(char **str, char **brl_options)
 {
-	if (!memcmp(*str, "brl,", 4)) {
+	if (!strncmp(*str, "brl,", 4)) {
 		*brl_options = "";
 		*str += 4;
-	} else if (!memcmp(str, "brl=", 4)) {
+	} else if (!strncmp(*str, "brl=", 4)) {
 		*brl_options = *str + 4;
 		*str = strchr(*brl_options, ',');
 		if (!*str)