NFS: add basic STATX_DIOALIGN and STATX_DIO_READ_ALIGN support

NFS: add basic STATX_DIOALIGN and STATX_DIO_READ_ALIGN support

NFS doesn't have DIO alignment constraints, so have NFS respond with
accommodating DIO alignment attributes (rather than plumb in GETATTR
support for STATX_DIOALIGN and STATX_DIO_READ_ALIGN).

The most coarse-grained dio_offset_align is the most accommodating
(e.g. PAGE_SIZE, in future larger may be supported).

Now that NFS has support, NFS reexport will now handle unaligned DIO
(NFSD's NFSD_IO_DIRECT support requires the underlying filesystem
support STATX_DIOALIGN and/or STATX_DIO_READ_ALIGN).

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