BTRFS hot reloc: add one hot reloc thread

   Add one private thread for hot relocation. It will check
if there're some extents which is hotter than the threshold
and queue them at first, if no, it will return and wait for
its next turn; otherwise, it will check if nonrotating disk
ratio is beyond its usage threshold, if no, it will directly
relocate hot extents from rotating disk to nonrotating disk;
otherwise it will find the extents with low temperature and
queue them, then relocate those extents with low temperature
and queue them, and finally relocate the hot extents from
from rotating disk to nonrotating disk.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
BTRFS hot reloc, procfs: add three proc interfaces

  Add three proc interfaces hot-reloc-interval, hot-reloc-threshold,
and hot-reloc-max-items under the dir /proc/sys/fs/ in order to
turn HOT_RELOC_INTERVAL, HOT_RELOC_THRESHOLD, and HOT_RELOC_MAX_ITEMS
into be tunable.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
BTRFS hot reloc: add hot relocation support

  Add one new mount option '-o hot_move' for hot
relocation support. When hot relocation is enabled,
hot tracking will be enabled automatically.
  Its usage looks like:
    mount -o hot_move
    mount -o nouser,hot_move
    mount -o nouser,hot_move,loop
    mount -o hot_move,nouser

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>