sysfs: remove signedness from sysfs_get_dirent

sysfs_get_dirent is usually invoked with a string literal, which
have the type char[].  While the toplevel Makefile
disables -Wpointer-sign, other Makefiles like

arch/x86/boot/compressed/Makefile

redefine KBUILD_CFLAGS. Fixes the warning:

In file included from arch/x86/boot/compressed/kaslr.c:17:
In file included from ./include/linux/module.h:17:
In file included from ./include/linux/kobject.h:21:
./include/linux/sysfs.h:517:37: warning: passing 'const unsigned char *'
to parameter of
      type 'const char *' converts between pointers to integer types
with different sign
      [-Wpointer-sign]
        return kernfs_find_and_get(parent, name);
                                           ^~~~
./include/linux/kernfs.h:462:57: note: passing argument to parameter
'name' here
kernfs_find_and_get(struct kernfs_node *kn, const char *name)
                                                        ^

Signed-off-by: Nick Desaulniers <nick.desaulniers@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>