ceph: fix potential use-after-free bug when trimming caps

When trimming the caps and just after the 'session->s_cap_lock' is
released in ceph_iterate_session_caps() the cap maybe removed by
another thread, and when using the stale cap memory in the callbacks
it will trigger use-after-free crash.

We need to check the existence of the cap just after the 'ci->i_ceph_lock'
being acquired. And do nothing if it's already removed.

Cc: stable@vger.kernel.org
Link: https://tracker.ceph.com/issues/43272


Signed-off-by: Xiubo Li <xiubli@redhat.com>
Reviewed-by: Luís Henriques <lhenriques@suse.de>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>