pnfs: add a new mechanism to select a layout driver according to an ordered list

Currently, the layout driver selection code always chooses the first one
from the list. That's not really ideal however, as the server can send
the list of layout types in any order that it likes. It's up to the
client to select the best one for its needs.

This patch adds an ordered list of preferred driver types and has the
selection code sort the list of available layout drivers according to it.
Any unrecognized layout type is sorted to the end of the list.

For now, the order of preference is hardcoded, but it should be possible
to make this configurable in the future.

Signed-off-by: Jeff Layton <jlayton@redhat.com>
Reviewed-by: J. Bruce Fields <bfields@fieldses.org>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

include/linux/nfs_xdr.h

@@ -144,6 +144,7 @@ struct nfs_fsinfo {
 	__u64			maxfilesize;
 	struct timespec		time_delta; /* server time granularity */
 	__u32			lease_time; /* in seconds */
+	__u32			nlayouttypes; /* number of layouttypes */
 	__u32			layouttype[NFS_MAX_LAYOUT_TYPES]; /* supported pnfs layout driver */
 	__u32			blksize; /* preferred pnfs io block size */
 	__u32			clone_blksize; /* granularity of a CLONE operation */