Btrfs: make flushoncommit mount option correctly wait on ordered_extents

The commit_transaction call to wait_ordered_extents when snap_pending
passes nocow_only=1 to process only NOCOW or PREALLOC extents.  This isn't
correct for the 'flushoncommit' mode, as it skips extents we just started
IO on in start_delalloc_inodes.

So, in the flushoncommit case, wait on all ordered extents.  Otherwise,
only pass the nocow_only flag to wait_ordered_extents if snap_pending.

Signed-off-by: Sage Weil <sage@newdream.net>
Signed-off-by: Chris Mason <chris.mason@oracle.com>