Btrfs: allocate extent state and check the result properly

It doesn't allocate extent_state and check the result properly:
- in set_extent_bit, it doesn't allocate extent_state if the path is not
  allowed wait

- in clear_extent_bit, it doesn't check the result after atomic-ly allocate,
  we trigger BUG_ON() if it's fail

- if allocate fail, we trigger BUG_ON instead of returning -ENOMEM since
  the return value of clear_extent_bit() is ignored by many callers

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>