ceph-for-4.10-rc1

A varied set of changes:

- a large rework of cephx auth code to cope with CONFIG_VMAP_STACK
  (myself).  Also fixed a deadlock caused by a bogus allocation on the
  writeback path and authorize reply verification.

- a fix for long stalls during fsync (Jeff Layton).  The client now
  has a way to force the MDS log flush, leading to ~100x speedups in
  some synthetic tests.

- a new [no]require_active_mds mount option (Zheng Yan).  On mount, we
  will now check whether any of the MDSes are available and bail rather
  than block if none are.  This check can be avoided by specifying the
  "no" option.

- a couple of MDS cap handling fixes and a few assorted patches
  throughout.
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