NFSD: Annotate struct pnfs_block_deviceaddr with __counted_by()

Add the __counted_by compiler attribute to the flexible array member
volumes to improve access bounds-checking via CONFIG_UBSAN_BOUNDS and
CONFIG_FORTIFY_SOURCE.

Use struct_size() instead of manually calculating the number of bytes to
allocate for a pnfs_block_deviceaddr with a single volume.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Acked-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>