btrfs: make lock_and_cleanup_extent_if_need nowait compatible

Add the nowait parameter to lock_and_cleanup_extent_if_need(). If the
nowait parameter is specified we try to lock the extent in nowait mode.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Stefan Roesch <shr@fb.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>