Btrfs: update the global reserve if it is empty

Before applying this patch, we reserved the space for the global reserve
by the minimum unit if we found it is empty, it was unreasonable and
inefficient, because if the global reserve space was depleted, it implied
that the size of the global reserve was too small. In this case, we shoud
update the global reserve and fill it.

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>