xfs: implement pNFS export operations

Add operations to export pNFS block layouts from an XFS filesystem.  See
the previous commit adding the operations for an explanation of them.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>

fs/xfs/xfs_pnfs.h

@@ -0,0 +1,11 @@
+#ifndef _XFS_PNFS_H
+#define _XFS_PNFS_H 1
+
+#ifdef CONFIG_NFSD_PNFS
+int xfs_fs_get_uuid(struct super_block *sb, u8 *buf, u32 *len, u64 *offset);
+int xfs_fs_map_blocks(struct inode *inode, loff_t offset, u64 length,
+		struct iomap *iomap, bool write, u32 *device_generation);
+int xfs_fs_commit_blocks(struct inode *inode, struct iomap *maps, int nr_maps,
+		struct iattr *iattr);
+#endif /* CONFIG_NFSD_PNFS */
+#endif /* _XFS_PNFS_H */