Instead of testing last_used (which could change in unusual circumstances) we
test against the bdev that we read frmo, and don't write back to there.

If two drives both fail during a write request, raid5 doesn't
cope properly and will eventually oops.

With this patch, blocks that have already been 'written'
are failed when double drive failure is noticed, as well as
blocks that are about to be written.

Sometimes raid0 and linear are required to take a single page bio that
spans two devices.  We use bio_split to split such a bio into two.

The the same time, bio.h is included by linux/raid/md.h so
we don't included it elsewhere anymore.

We also modify the mergeable_bvec functions to allow a bvec
that doesn't fit if it is the first bvec to be added to
the bio, and be careful never to return a negative length from a
bvec_mergable funciton.

OK, that's it - no callout drivers left, so we drop the warning from
tty_open() and kill the code that handles them in /proc/tty/drivers.

misc cleanup - switched to standard constant names for .subtype in the
drivers that had private equivalents, removed unused ->callout_termios
and friends from the last places where they were defined.

callout removal: isdn

callout removal: cris

callout removal: simserial

callout removal: mips

callout removal: vacserial

callout removal: sicc

callout removal: 8260_uart

callout removal: 8xx_uart

callout removal: amiserial

callout removal: cyclades

callout removal: dz

callout removal: epca

callout removal: esp

callout removal: ip2

callout removal: moxa

callout removal: mxser

callout removal: pcxx

callout removal: rocket

callout removal: serial167

callout removal: synclink

callout removal: synclinkmp

callout removal: synclink_cs

callout removal: macserial

callout removal: pc300

callout removal: chdlc

callout removal: 68328

callout removal: isicom

callout removal: generic_serial

callout removal: tx3912

callout removal: vme

callout removal: sci

callout removal: istallion

callout removal: riscom8