Btrfs: don't steal the reserved space from the global reserve if their space type is different

If the type of the space we need is different with the global reserve, we
can not steal the space from the global reserve, because we can not allocate
the space from the free space cache that the global reserve points to.

Cc: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>