btrfs: push errors up from add_async_extent()

[ Upstream commit dbe6cda6 ]

The memory allocation error in add_async_extent() is not handled
properly, return an error and push the BUG_ON to the caller. Handling it
there is not trivial so at least make it visible.

Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>