A new VFS-module which uses libcephfs' low-level APIs (instead of the
higher-level APIs used by current vfs_ceph.c module) to communicate
with an existing cephfs cluster. Using the low-level APIs allows more
fine-grained functionality:

  - Explicit user-credentials (libcephfs' UserPerms) per call.
  - Faster operation via (cached) inode or file-handle.
  - Improved resource consumption on libcephfs internal Client.
  - Asynchronous I/O (ceph_ll_nonblocking_readv_writev, not impl yet).

The code uses iget/iput pattern: resolve ("iget") a reference to Ceph's
Inode by inode-number (fast) or path-resolving (slow) at the beginning
of operation, execute a specific call using this Inode, and release it
at the end ("iput").

When calling specific low-level APIs, create an appropriate Ceph
UserPerm object on-the-fly based on current unix_token credentials
(uid, gid and supplementary groups).

For I/O operation use fsp-extension mechanism to keep the returned
file-handle reference (Ceph's Fh*) associated with the relevant
files_struct.

Signed-off-by: Shachar Sharon <ssharon@redhat.com>