proc: add two proc interfaces

  Add two proc interfaces hot-age-interval and hot-update-interval
under the dir /proc/sys/fs/ in order to turn HOT_AGE_INTERVAL and
HOT_UPDATE_INTERVAL into be tunable.

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

  Introduce one new mount option '-o hot_track',
and add its parsing support.
  Its usage looks like:
   mount -o hot_track
   mount -o nouser,hot_track
   mount -o nouser,hot_track,loop
   mount -o hot_track,nouser

Reviewed-by:   David Sterba <dsterba@suse.cz>
Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: add documentation

  Add Documentation for VFS hot tracking feature

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: add fs hot type support

  Introduce one ability to enable that specific FS
can register its own hot tracking functions.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
vfs: add one list_head field

  Add one list_head field 'reloc_list' to accommodate
hot relocation support.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
btrfs: Introduce one new block group

  Introduce one new block group BTRFS_BLOCK_GROUP_DATA_SSD,
which is used to differentiate if the block space is reserved
and allocated from one HDD disk or SSD disk.

Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
btrfs: add one task for hot relocation

   Add the task logic for hot relocation.

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