btrfs: Simplify btrfs_submit_bio_hook

btrfs_submit_bio_hook currently calls btrfs_bio_wq_end_io in either case
of an if statement that determines one of the arguments.

This patch moves the function call outside of the if statement and uses it
to only determine the different argument. This allows us to catch an
error in one place in a more visually obvious way.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>