ceph: cleanup return error of try_get_cap_refs()

Returns 0 if caps were not able to be acquired (yet), 1 if cap
acquisition succeeded, or a negative error code. There are 3 special
error codes:

-EAGAIN: need to sleep but non-blocking is specified
-EFBIG:  ask caller to call check_max_size() and try again.
-ESTALE: ask caller to call ceph_renew_caps() and try again.

[ jlayton: add WARN_ON_ONCE check for -EAGAIN ]

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>