mds: add ephemeral pinning for subtrees

mds: add ephemeral pinning for subtrees

This PR introduces inode xattrs export_ephemeral_random and
export_ephemeral_distributed which enables two different metadata
distribution strategies - the first being suitable for a more depthwise
scaling of metadata (height of the tree keeps increasing) and the latter
for horizontal scaling (many subtrees under a single parent).
export_ephemeral_distributed applies is not hierarchical. Any direct
descendant directory (i.e. a child directory) has an ephemeral export
pin applied to it according to a consistent hash of the child directory
inode number. export_ephemeral_distributed is hierarchical like
"export_pin". Any CDir loaded into the cache may be ephemerally pinned
to a random rank. Like "export_ephemeral_distributed", the random rank
is determined by a consistent hash.

The metadata distribution strategies are facilitated by using John
Lamping and Eric Veach's Jump Consistent Hashing as the consistent hash
algorithm. This hashing algorithm eliminates the need to store the data
structures representing the consistent hash cluster state and performs
as well as Akamai's original implementation providing a fairly uniform
distribution. This algorithm only works for distributed systems with
numbered buckets (nodes) arranged in ascending order and cluster resizes
does not produce any holes in the arrangement of nodes i.e (0, 1, 2, 3)
--[removing node 1]--> (0, 1, 2). CephFS satisfies these conditions as
the MDSs are arranged as numbered ranks and cluster modifications does
not produce any holes in the resulting arrangement of ranks.

Fixes: https://tracker.ceph.com/issues/41302
Signed-off-by: Sidharth Anupkrishnan <sanupkri@redhat.com>
Signed-off-by: Patrick Donnelly <pdonnell@redhat.com>