btrfs: add one hot relocation kthread

   Add one private kthread 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 SSD ratio is beyond
beyond its usage threshold, if no, it will directly relocate
those hot extents from HDD disk to SSD 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 HDD disk to SSD
disk.

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