nfs/localio: Ensure DIO WRITE's IO on stable storage upon completion

nfs/localio: Ensure DIO WRITE's IO on stable storage upon completion

LOCALIO's misaligned DIO WRITE support requires synchronous IO for any
misaligned head and/or tail that are issued using buffered IO.  In
addition, it is important that the O_DIRECT middle be on stable
storage upon its completion via AIO.

Otherwise, a misaligned DIO WRITE could mix buffered IO for the
head/tail and direct IO for the DIO-aligned middle -- which could lead
to problems associated with deferred writes to stable storage (such as
out of order partial completions causing incorrect advancement of the
file's offset, etc).

Fixes: c817248fc831 ("nfs/localio: add proper O_DIRECT support for READ and WRITE")
Signed-off-by: Mike Snitzer <snitzer@kernel.org>
Signed-off-by: Anna Schumaker <anna.schumaker@oracle.com>