btrfs: scrub: handle RST lookup error correctly

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

btrfs: scrub: handle RST lookup error correctly

[BUG]

When running btrfs/060 with forced RST feature, it would crash the

following ASSERT() inside scrub_read_endio():

ASSERT(sector_nr < stripe->nr_sectors);

Before that, we would have tree dump from

btrfs_get_raid_extent_offset(), as we failed to find the RST entry for

the range.

[CAUSE]

Inside scrub_submit_extent_sector_read() every time we allocated a new

bbio we immediately called btrfs_map_block() to make sure there was some

RST range covering the scrub target.

But if btrfs_map_block() fails, we immediately call endio for the bbio,

while the bbio is newly allocated, it's completely empty.

Then inside scrub_read_endio(), we go through the bvecs to find

the sector number (as bi_sector is no longer reliable if the bio is

submitted to lower layers).

And since the bio is empty, such bvecs iteration would not find any

sector matching the sector, and return sector_nr == stripe->nr_sectors,

triggering the ASSERT().

[FIX]

Instead of calling btrfs_map_block() after allocating a new bbio, call

btrfs_map_block() first.

Since our only objective of calling btrfs_map_block() is only to update

stripe_len, there is really no need to do that after btrfs_alloc_bio().

This new timing would avoid the problem of handling empty bbio

completely, and in fact fixes a possible race window for the old code,

where if the submission thread is the only owner of the pending_io, the

scrub would never finish (since we didn't decrease the pending_io

counter).

Although the root cause of RST lookup failure still needs to be

addressed.