nfs/localio: add proper O_DIRECT support for READ and WRITE

nfs/localio: add proper O_DIRECT support for READ and WRITE

Because the NFS client will already happily handle misaligned O_DIRECT
IO (by sending it out to NFSD via RPC) this commit's new capabilities
are for the benefit of LOCALIO.

LOCALIO will make best effort to transform misaligned IO to
DIO-aligned extents when possible.

LOCALIO's READ and WRITE DIO that is misaligned will be split into as
many as 3 component IOs (@start, @middle and @end) as needed -- IFF
the @middle extent is verified to be DIO-aligned, and then the @start
and/or @end are misaligned (due to each being a partial page).
Otherwise if the @middle isn't DIO-aligned the code will fallback to
issuing only a single contiguous buffered IO.

The @middle is only DIO-aligned if both the memory and on-disk offsets
for the IO are aligned relative to the underlying local filesystem's
block device limits (@dma_alignment and @logical_block_size
respectively).

The misaligned @start and/or @end extents are issued using buffered IO
and the DIO-aligned @middle is issued using O_DIRECT. The @start and
@end IOs are issued first using buffered IO with IOCB_SYNC and then
the @middle is issued last using direct IO with async completion (AIO).
This out of order IO completion means that LOCALIO's IO completion
code (nfs_local_read_done and nfs_local_write_done) is only called for
the IO's last associated iov_iter completion. And in the case of
DIO-aligned @middle it completes last using AIO. nfs_local_pgio_done()
is updated to handle piece-wise partial completion of each iov_iter.

This implementation for LOCALIO's misaligned DIO handling uses 3
iov_iter that share the same backing pages in their bio_vecs (so
unfortunately 'struct nfs_local_kiocb' has 3 instead of only 1).

[Reducing LOCALIO's per-IO (struct nfs_local_kiocb) memory use can be
explored in the future. One logical progression to improve this code,
and eliminate explicit loops over up to 3 iov_iter, is by extending
'struct iov_iter' to support iov_iter_clone() and iov_iter_chain()
interfaces that are comparable to what 'struct bio' is able to support
in the block layer. But even that wouldn't avoid the need to
allocate/use up to 3 iov_iter]

Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>