Btrfs: don't look at bio flags after submit_bio

After callling submit_bio, the bio can be freed at any time.  The
btrfs submission thread helper was checking the bio flags too late,
which might not give the correct answer.

When CONFIG_DEBUG_PAGE_ALLOC is turned on, it can lead to oopsen.

Signed-off-by: Chris Mason <chris.mason@oracle.com>