Btrfs: free sys_array eb as soon as possible

While reading sys_chunk_array in superblock, btrfs creates a temporary
extent buffer.  Since we don't use it after finishing reading
 sys_chunk_array, we don't need to keep it in memory.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>