ceph: don't take i_ceph_lock in handle_cap_import

Just take it before calling it. This means we have to do a couple of
minor in-memory operations under the spinlock now, but those shouldn't
be an issue.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>