client: void sending mds request while holding cap reference

client: void sending mds request while holding cap reference

Client::_write() calls is_quota_bytes_approaching() while holding Fw
cap reference. The later function may send lookup_name request to mds.
This can cause deadlock (mds delays handling the lookup_name request
because subtree is freezing. The subtree stays in freezing state because
mds is revoking the Fw cap and client does not release it)

This patch isn't cherry-picked from master because quota implementation
has been changed in master (introduced quota realm).

Fixes: http://tracker.ceph.com/issues/24369
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>