blk-mq: Fix spelling in a source code comment

Change one occurrence of 'performace' into 'performance'.

Cc: Max Gurtovoy <maxg@mellanox.com>
Fixes: fe631457

 ("blk-mq: map all HWQ also in hyperthreaded system") # v4.13.
Reviewed-by: Chaitanya Kulkarni <chiatanya.kulkarni@wdc.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>