vfs: mnt_drop_write_file()

new helper (wrapper around mnt_drop_write()) to be used in pair with
mnt_want_write_file().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/xfs/xfs_ioctl32.c

@@ -461,7 +461,7 @@ xfs_compat_attrmulti_by_handle(
 					dentry->d_inode, attr_name,
 					compat_ptr(ops[i].am_attrvalue),
 					ops[i].am_length, ops[i].am_flags);
-			mnt_drop_write(parfilp->f_path.mnt);
+			mnt_drop_write_file(parfilp);
 			break;
 		case ATTR_OP_REMOVE:
 			ops[i].am_error = mnt_want_write_file(parfilp);
@@ -470,7 +470,7 @@ xfs_compat_attrmulti_by_handle(
 			ops[i].am_error = xfs_attrmulti_attr_remove(
 					dentry->d_inode, attr_name,
 					ops[i].am_flags);
-			mnt_drop_write(parfilp->f_path.mnt);
+			mnt_drop_write_file(parfilp);
 			break;
 		default:
 			ops[i].am_error = EINVAL;