Reduce priviliges required to query btrfs volumes

[leth]

In prometheus/node_exporter#2632 we figured out that O_NOATIME was increasing the privileges needed!

From the open man page

O_NOATIME (since Linux 2.6.8)
              Do not update the file last access time (st_atime in the
              inode) when the file is [read(2)](https://man7.org/linux/man-pages/man2/read.2.html).

              This flag can be employed only if one of the following
              conditions is true:

              *  The effective UID of the process matches the owner UID
                 of the file.

              *  The calling process has the CAP_FOWNER capability in
                 its user namespace and the owner UID of the file has a
                 mapping in the namespace.

[dennwc]

Awesome, thank you!