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/ext4/ioctl.c b/fs/ext4/ioctl.c
index 9a49760..d37b3bb 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -134,7 +134,7 @@
 			err = ext4_ext_migrate(inode);
 flags_out:
 		mutex_unlock(&inode->i_mutex);
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		return err;
 	}
 	case EXT4_IOC_GETVERSION:
@@ -171,7 +171,7 @@
 		}
 		ext4_journal_stop(handle);
 setversion_out:
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		return err;
 	}
 	case EXT4_IOC_GROUP_EXTEND: {
@@ -204,7 +204,7 @@
 		}
 		if (err == 0)
 			err = err2;
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		ext4_resize_end(sb);
 
 		return err;
@@ -246,7 +246,7 @@
 
 		err = ext4_move_extents(filp, donor_filp, me.orig_start,
 					me.donor_start, me.len, &me.moved_len);
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		if (me.moved_len > 0)
 			file_remove_suid(donor_filp);
 
@@ -289,7 +289,7 @@
 		}
 		if (err == 0)
 			err = err2;
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		ext4_resize_end(sb);
 
 		return err;
@@ -313,7 +313,7 @@
 		mutex_lock(&(inode->i_mutex));
 		err = ext4_ext_migrate(inode);
 		mutex_unlock(&(inode->i_mutex));
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		return err;
 	}
 
@@ -327,7 +327,7 @@
 		if (err)
 			return err;
 		err = ext4_alloc_da_blocks(inode);
-		mnt_drop_write(filp->f_path.mnt);
+		mnt_drop_write_file(filp);
 		return err;
 	}