btrfs: Use schedule_timeout_interruptible

Instead of manually fiddling with the state of the task
(RUNNING->INTERRUPTIBLE->RUNNING) again just use schedule_timeout_interruptible
which adjusts the task state as needed. No functional changes.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Josef Bacik <jbacik@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>