Linux compat 4.16: blk_queue_flag_{set,clear}

queue_flag_{set,clear}_unlocked are now private interfaces in
the Linux kernel (https://github.com/torvalds/linux/commit/8a0ac14).
Use blk_queue_flag_{set,clear} interfaces which were introduced as
of https://github.com/torvalds/linux/commit/8814ce8.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Closes #7410