Fix the pNFS server's reporting of SpaceUsed (va_bytes).

The pNFS server currently reports SpaceUsed (va_bytes) for the metadata
file. This in not correct, since the metadata file is always empty and,
as such, va_bytes is just the allocation for the empty file.
This patch adds va_bytes to the list of attributes acquired from the
DS for a file, so that it includes the allocated data size and is updated
when the file is written.
For files created on a pNFS server before this patch is applied, the
va_bytes value is estimated by rounding va_size up to a multiple of
BLKDEV_IOSIZE. Once the file is written after this patch has been
applied to the metadata server, the va_bytes returned for the file
will be correct.

This patch only affects a pNFS metadata server.

Found during testing of the NFSv4.2 pNFS server for the Allocate operation.
(Not yet in head/current.)

MFC after: 2 weeks