cephfs: fix write_buf's _len overflow problem

cephfs: fix write_buf's _len overflow problem

After I have set about 400 64KB xattr kv pair to a file,
mds is crashed. Every time I try to start mds, it will crash again.
The root reason is write_buf._len overflowed when doing
Journaler::append_entry().

This patch try to fix this problem through the following changes:

 1. limit file/dir's xattr size
 2. throttle journal entry append operations

Fixes: http://tracker.ceph.com/issues/19033
Signed-off-by: Yang Honggang joseph.yang@xtaotech.com