Btrfs: revert checksum error statistic which can cause a BUG()

Btrfs: revert checksum error statistic which can cause a BUG()

Commit 9bc01bd6326ae4dce50ceb45810573c4c476e6ae added btrfs device
statistic counters for detected IO and checksum errors to Linux 3.5.
The statistic part that counts checksum errors in
end_bio_extent_readpage() can cause a BUG() in a subfunction:
"kernel BUG at fs/btrfs/volumes.c:3762!"
That part is reverted with the current patch.
However, the counting of checksum errors in the scrub context remains
active, and the counting of detected IO errors (read, write or flush
errors) in all contexts remains active.

Cc: stable <stable@vger.kernel.org> # 3.5
Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
Signed-off-by: Chris Mason <chris.mason@oracle.com>