rbd: spell out image features unsupported by the kernel

rbd: spell out image features unsupported by the kernel

/sys/bus/rbd/supported_features is available on kernels starting with
4.11.  Example output:

    $ sudo rbd map x
    rbd: sysfs write failed
    RBD image feature set mismatch. This image cannot be mapped because the following immutable features are unsupported by the kernel: data-pool.
    In some cases useful info is found in syslog - try "dmesg | tail".
    rbd: map failed: (6) No such device or address

    $ sudo rbd map -p foo y
    rbd: sysfs write failed
    RBD image feature set mismatch. You can disable features unsupported by the kernel with "rbd feature disable foo/y object-map fast-diff deep-flatten".
    In some cases useful info is found in syslog - try "dmesg | tail".
    rbd: map failed: (6) No such device or address

or, if /sys/bus/rbd/supported_features isn't there:

    $ sudo rbd map z
    rbd: sysfs write failed
    RBD image feature set mismatch. Try disabling features unsupported by the kernel with "rbd feature disable".
    In some cases useful info is found in syslog - try "dmesg | tail".
    rbd: map failed: (6) No such device or address

Fixes: http://tracker.ceph.com/issues/19095
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>