btrfs-progs: add hot relocation support in mkfs

  Introduce one new mkfs.btrfs option (-h) for hot relocation
which preallocate two new block groups BTRFS_BLOCK_GROUP_DATA_SSD
and BTRFS_BLOCK_GROUP_METADATA_SSD, and the latter is planned for
future use by hot relocation.

Signed-off-by: Ben Chociej <bchociej@gmail.com>
Signed-off-by: Matt Lupfer <mlupfer@gmail.com>
Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>