btrfs scrub - make fixups sync, don't reuse fixup bios

Fixups are already sync for csum failures, this patch makes them sync
for EIO case as well.

Fixups are now sharing pages with the parent sbio - instead of
allocating a separate page to do a fixup we grab the page from the sbio
buffer.

Fixup bios are no longer reused.

struct fixup is no longer needed, instead pass [sbio pointer, index].

Originally this was added to look at the possibility of sharing the code
between drive swap and scrub, but it actually fixes a serious bug in
scrub code where errors that could be corrected were ignored and
reported as uncorrectable.

btrfs scrub - restore bios properly after media errors

The current code reallocates a bio after a media error.  This is a
temporary measure introduced in v3 after a serious problem related to
bio reuse was found in v2 of scrub patchset.

Basically we did not reset bv_offset and bv_len fields of the bio_vec
structure.  They are changed in case I/O error happens, for example, at
offset 512 or 1024 into the page.  Also bi_flags field wasn't properly
setup before reusing the bio.
Signed-off-by: NArne Jansen <sensille@gmx.net>

This adds an initial implementation for scrub. It works quite
straightforward. The usermode issues an ioctl for each device in the
fs. For each device, it enumerates the allocated device chunks. For
each chunk, the contained extents are enumerated and the data checksums
fetched. The extents are read sequentially and the checksums verified.
If an error occurs (checksum or EIO), a good copy is searched for. If
one is found, the bad copy will be rewritten.
All enumerations happen from the commit roots. During a transaction
commit, the scrubs get paused and afterwards continue from the new
roots.

This commit is based on the series originally posted to linux-btrfs
with some improvements that resulted from comments from David Sterba,
Ilya Dryomov and Jan Schmidt.
Signed-off-by: NArne Jansen <sensille@gmx.net>