futex: Fix spelling in a source code comment

Change a single occurrence of "unlcoked" into "unlocked".

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Cc: Darren Hart <dvhltc@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>