btrfs: fix incorrect splitting in btrfs_drop_extent_map_range

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix incorrect splitting in btrfs_drop_extent_map_range

In production we were seeing a variety of WARN_ON()'s in the extent_map
code, specifically in btrfs_drop_extent_map_range() when we have to call
add_extent_mapping() for our second split.

Consider the following extent map layout

PINNED
[0 16K) [32K, 48K)

and then we call btrfs_drop_extent_map_range for [0, 36K), with
skip_pinned == true. The initial loop will have

start = 0
end = 36K
len = 36K

we will find the [0, 16k) extent, but since we are pinned we will skip
it, which has this code

start = em_end;
if (end != (u64)-1)
len = start + len - em_end;

em_end here is 16K, so now the values are

start = 16K
len = 16K + 36K - 16K = 36K

len should instead be 20K. This is a problem when we find the next
extent at [32K, 48K), we need to split this extent to leave [36K, 48k),
however the code for the split looks like this

split->start = start + len;
split->len = em_end - (start + len);

In this case we have

em_end = 48K
split->start = 16K + 36K // this should be 16K + 20K
split->len = 48K - (16K + 36K) // this overflows as 16K + 36K is 52K

and now we have an invalid extent_map in the tree that potentially
overlaps other entries in the extent map. Even in the non-overlapping
case we will have split->start set improperly, which will cause problems
with any block related calculations.

We don't actually need len in this loop, we can simply use end as our
end point, and only adjust start up when we find a pinned extent we need
to skip.

Adjust the logic to do this, which keeps us from inserting an invalid
extent map.

We only skip_pinned in the relocation case, so this is relatively rare,
except in the case where you are running relocation a lot, which can
happen with auto relocation on.