ceph: fix special error code in ceph_try_get_caps()

There are 3 speical error codes: -EAGAIN/-EFBIG/-ESTALE.
After calling try_get_cap_refs, ceph_try_get_caps test for the
-EAGAIN twice. Ensure that it tests for -ESTALE instead.

Signed-off-by: Wu Bo <wubo40@huawei.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>