nfs: setattr can only change regular file sizes

The VFS never calls setattr with ATTR_SIZE on anything but regular
files.  Remove the if check and turn it into an assert similar to
what some other file systems do.
Signed-off-by: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NTrond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/inode.c

@@ -505,7 +505,9 @@ nfs_setattr(struct dentry *dentry, struct iattr *attr)
 		attr->ia_valid &= ~ATTR_MODE;
 
 	if (attr->ia_valid & ATTR_SIZE) {
-		if (!S_ISREG(inode->i_mode) || attr->ia_size == i_size_read(inode))
+		BUG_ON(!S_ISREG(inode->i_mode));
+
+		if (attr->ia_size == i_size_read(inode))
 			attr->ia_valid &= ~ATTR_SIZE;
 	}