Merge remote-tracking branch 'remotes/kevin/tags/for-upstream' into staging

Block layer fixes for 2.9.0-rc1

# gpg: Signature made Fri 17 Mar 2017 12:06:04 GMT
# gpg:                using RSA key 0x7F09B272C88F2FD6
# gpg: Good signature from "Kevin Wolf <kwolf@redhat.com>"
# Primary key fingerprint: DC3D EB15 9A9A F95D 3D74  56FE 7F09 B272 C88F 2FD6

* remotes/kevin/tags/for-upstream:
  block: quiesce AioContext when detaching from it
  thread-pool: add missing qemu_bh_cancel in completion function
  block: Propagate error in bdrv_open_backing_file
  blockdev: fix bitmap clear undo
  block: Always call bdrv_child_check_perm first
  file-posix: Don't leak fd in hdev_get_max_segments
  replication: clarify permissions
  file-posix: clean up max_segments buffer termination

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>