ceph: use atomic_t for ceph_inode_info::i_shared_gen

ceph: use atomic_t for ceph_inode_info::i_shared_gen

BugLink: https://bugs.launchpad.net/bugs/1834235
It allows accessing i_shared_gen without holding i_ceph_lock. It is
preparation for later patch.

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 97aeb6bf988e0830fd80dca724fd89526b3f35e4)
Signed-off-by: Connor Kuehl <connor.kuehl@canonical.com>
Acked-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>