btrfs-tools: upgrade 6.0.2 -> 6.1

Changelog:
==========
* filesystem df: add json output
* qgroup show: add json output
* new command: 'inspect-internal map-swapfile' to check swapfile and its
  swapfile_offset value used for hibernation
* corrupt-block: fix parsing of option --root argument
* experimental (interfaces not finalized):
   * new command 'inspect-internal list-chunks'
   * new group reflink, command clone
* other:
   * synchronize some files with kernel versions
   * docs updates
   * build: use gnu11

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>