mm/util.c: add kstrimdup()

mm/util.c: add kstrimdup()

kstrimdup() creates a whitespace-trimmed duplicate of the passed in
null-terminated string.  This is useful for strings coming from sysfs that
often include trailing whitespace due to user input.

Thanks to Joe Perches for this implementation.

Signed-off-by: Sebastian Capella <sebastian.capella@linaro.org>
Cc: Joe Perches <joe@perches.com>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>