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>
diff --git a/fs/ext3/ioctl.c b/fs/ext3/ioctl.c
index a02863a..8e37c41 100644
--- a/fs/ext3/ioctl.c
+++ b/fs/ext3/ioctl.c
@@ -110,7 +110,7 @@
 			err = ext3_change_inode_journal_flag(inode, jflag);
 flags_out:
 		mutex_unlock(&inode->i_mutex);
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		return err;
 	}
 	case EXT3_IOC_GETVERSION:
@@ -147,7 +147,7 @@
 		}
 		ext3_journal_stop(handle);
 setversion_out:
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		return err;
 	}
 	case EXT3_IOC_GETRSVSZ:
@@ -195,7 +195,7 @@
 		}
 		mutex_unlock(&ei->truncate_mutex);
 setrsvsz_out:
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		return err;
 	}
 	case EXT3_IOC_GROUP_EXTEND: {
@@ -221,7 +221,7 @@
 		if (err == 0)
 			err = err2;
 group_extend_out:
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		return err;
 	}
 	case EXT3_IOC_GROUP_ADD: {
@@ -249,7 +249,7 @@
 		if (err == 0)
 			err = err2;
 group_add_out:
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		return err;
 	}
 	case FITRIM: {