block: constify ext_pi_ref_escape()

This function doesn't mutate data.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/d24611b3-dddf-473a-903d-39290db03b11@p183


Signed-off-by: Jens Axboe <axboe@kernel.dk>