iov_iter: hide iovec details behind ops function pointers

From: Zach Brown <zab@zabbo.net>

This moves the current iov_iter functions behind an ops struct of
function pointers.  The current iov_iter functions all work with memory
which is specified by iovec arrays of user space pointers.

This patch is part of a series that lets us specify memory with bio_vec
arrays of page pointers.  By moving to an iov_iter operation struct we
can add that support in later patches in this series by adding another
set of function pointers.

I only came to this after having initialy tried to teach the current
iov_iter functions about bio_vecs by introducing conditional branches
that dealt with bio_vecs in all the functions.  It wasn't pretty.  This
approach seems to be the lesser evil.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Cc: Zach Brown <zab@zabbo.net>
iov_iter: add bvec support

From: Zach Brown <zab@zabbo.net>

This adds a set of iov_iter_ops calls which work with memory which is
specified by an array of bio_vec structs instead of an array of iovec
structs.

The big difference is that the pages referenced by the bio_vec elements
are pinned.  They don't need to be faulted in and we can always use
kmap_atomic() to map them one at a time.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Cc: Zach Brown <zab@zabbo.net>
iov_iter: add a shorten call

From: Zach Brown <zab@zabbo.net>

The generic direct write path wants to shorten its memory vector.  It
does this when it finds that it has to perform a partial write due to
LIMIT_FSIZE.  .direct_IO() always performs IO on all of the referenced
memory because it doesn't have an argument to specify the length of the
IO.

We add an iov_iter operation for this so that the generic path can ask
to shorten the memory vector without having to know what kind it is.
We're happy to shorten the kernel copy of the iovec array, but we refuse
to shorten the bio_vec array and return an error in this case.

Signed-off-by: Dave Kleikamp <dave.kleikamp@oracle.com>
Cc: Zach Brown <zab@zabbo.net>