We're going to support partial extent moving, which may split entire extent
movement into pieces to compromise the insuffice allocations, it eases the
'ENSPC' pain and makes the whole moving much less likely to fail, the downside
is it may make the fs even more fragmented before moving, just let the userspace
make a trade-off here.
Signed-off-by: Tristan Ye <tristan.ye@oracle.com>