mds: continue linking if targeti is temporarily located in stray dir

mds: continue linking if targeti is temporarily located in stray dir

If one inode has more than one hardlink and after the primary dentry
is unlinked it will located the inode in the stray dir temporarily,
which is pending reintegration.

Just before the linkmerge/migrate is triggered a link request comes
it will fail with -EXDEV.

Just skip it and continue the linking.

Fixes: https://tracker.ceph.com/issues/56695
Signed-off-by: Xiubo Li <xiubli@redhat.com>