nfs41: change PNFS_LAYOUTRET_ON_SETATTR to only return on truncation to smaller size

Both blocks layout and objects layout want to use it to avoid CB_LAYOUTRECALL
but that should only happen if client is doing truncation to a smaller size.
For other cases, we let server decide if it wants to recall client's layouts.
Change PNFS_LAYOUTRET_ON_SETATTR to follow the logic and not to send
layoutreturn unnecessarily.

Cc: Christoph Hellwig <[e-mail]>
Cc: Boaz Harrosh <[e-mail]>
Signed-off-by: Peng Tao <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/nfs4proc.c

···
 	struct nfs4_label *label = NULL;
 	int status;
 
-	if (pnfs_ld_layoutret_on_setattr(inode))
+	if (pnfs_ld_layoutret_on_setattr(inode) &&
+	    sattr->ia_valid & ATTR_SIZE &&
+	    sattr->ia_size < i_size_read(inode))
 		pnfs_commit_and_return_layout(inode);
 
 	nfs_fattr_init(fattr);

fs/nfs/pnfs.h

···
 };
 
 enum layoutdriver_policy_flags {
-	/* Should the pNFS client commit and return the layout upon a setattr */
+	/* Should the pNFS client commit and return the layout upon truncate to
+	 * a smaller size */
 	PNFS_LAYOUTRET_ON_SETATTR	= 1 << 0,
 	PNFS_LAYOUTRET_ON_ERROR		= 1 << 1,
 	PNFS_READ_WHOLE_PAGE		= 1 << 2,