If we are heavily fragmented we will continually try to prealloc the largest
extent size we can every time we call btrfs_reserve_extent.  This can be very
expensive when we are heavily fragmented, burning lots of CPU cycles and loops
through the allocator.  So instead notice when we get a smaller chunk from the
allocator than what we specified and use this as the new maximum size we try to
allocate.  Thanks,
Signed-off-by: NJosef Bacik <jbacik@fb.com>
Signed-off-by: NChris Mason <clm@fb.com>