os/FileStore: print out BTRFS_IOC_SNAP_CREATE errs
os/FileStore: print out BTRFS_IOC_SNAP_CREATE errs

Print out the errno for BTRFS_IOC_SNAP_CREATE errors if they happen.

Signed-off-by: Colin McCabe <7b35906f1d014e74a9dc8c913eb229119df647aa@dreamhost.com>

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph