RBD: disable and enable features depending on kernel version

RBD: disable and enable features depending on kernel version

Modern kernel, like 5.3, support all those features ('fast-diff',
'object-map', 'deep-flatten'), so we do not want to disable them
there. 5.0 already supports exclusive-locks, so no need to disable
exclusive locking there.

Further, we also want to profit from new features available, so let's
enable those which can be enabled "live" (i.e., after image creation)
if their available.

While we could also parse the kernel information directly from:
/sys/module/libceph/parameters/supported_features
there's not much advantage to that, features cannot be disabled with
KConfig, their also very dependent of the kernel version booted.
So for us it's enough to check that one.

This only affects container and VMs backed by a storage with KRBD
explicitly enabled. But as the enabling and disabling happens
transparently, it has no effect on the running guest.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>