VFS hot tracking: Add documentation

Add Documentation for VFS hot tracking feature

Signed-off-by: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
VFS hot tracking, 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: Chandra Seetharaman <sekharan@us.ibm.com>
Signed-off-by: Zhi Yong Wu <wuzhy@linux.vnet.ibm.com>
VFS hot tracking, xfs: Add hot tracking support

Connect up the VFS hot tracking support so XFS filesystem
can make use of it.

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