ceph: support copy_file_range file operation

This commit implements support for the copy_file_range syscall in cephfs.
It is implemented using the RADOS 'copy-from' operation, which allows to
do a remote object copy, without the need to download/upload data from/to
the OSDs.

Some manual copy may however be required if the source/destination file
offsets aren't object aligned or if the copy length is smaller than the
object size.

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>