Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-05-04' into staging

Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-05-04' into staging

nbd patches for 2020-05-04

- reduce client-side fragmentation of NBD trim and status requests
- fix iotest 41 when run in deep tree
- fix socket activation in qemu-nbd

# gpg: Signature made Mon 04 May 2020 22:12:21 BST
# gpg:                using RSA key 71C2CC22B1C4602927D2F3AAA7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>" [full]
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>" [full]
# gpg:                 aka "[jpeg image of size 6874]" [full]
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-nbd-2020-05-04:
  block/nbd-client: drop max_block restriction from discard
  block/nbd-client: drop max_block restriction from block_status
  iotests/041: Fix NBD socket path
  tools: Fix use of fcntl(F_SETFD) during socket activation

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