Btrfs: fix unprotected system chunk array insertion

We didn't protect the system chunk array when we added a new
system chunk into it, it would cause the array be corrupted
if someone remove/add some system chunk into array at the same
time. Fix it by chunk lock.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>