ceph: don't return -ESTALE if there's still an open file

Similarly to commit 03f21904 ("ceph: check i_nlink while converting
a file handle to dentry"), this fixes another corner case with
name_to_handle_at/open_by_handle_at.  The issue has been detected by
xfstest generic/467, when doing:

 - name_to_handle_at("/cephfs/myfile")
 - open("/cephfs/myfile")
 - unlink("/cephfs/myfile")
 - sync; sync;
 - drop caches
 - open_by_handle_at()

The call to open_by_handle_at should not fail because the file hasn't been
deleted yet (only unlinked) and we do have a valid handle to it.  -ESTALE
shall be returned only if i_nlink is 0 *and* i_count is 1.

This patch also makes sure we have LINK caps before checking i_nlink.

Signed-off-by: Luis Henriques <lhenriques@suse.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Acked-by: Amir Goldstein <amir73il@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>