blk-wbt: pass in enum wbt_flags to get_rq_wait()

blk-wbt: pass in enum wbt_flags to get_rq_wait()

BugLink: https://bugs.launchpad.net/bugs/1810998
This is in preparation for having more write queues, in which
case we would have needed to pass in more information than just
a simple 'is_kswapd' boolean.

Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
(cherry picked from commit 8bea60901974ad44b06b08d52e1dd421ea8c6e9c)
Signed-off-by: Mauricio Faria de Oliveira <mfo@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>