switch a bunch of places to mnt_want_write_file()

it's both faster (in case when file has been opened for write) and cleaner.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index a567968..9a49760 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -45,7 +45,7 @@
 		if (get_user(flags, (int __user *) arg))
 			return -EFAULT;
 
-		err = mnt_want_write(filp->f_path.mnt);
+		err = mnt_want_write_file(filp);
 		if (err)
 			return err;
 
@@ -150,7 +150,7 @@
 		if (!inode_owner_or_capable(inode))
 			return -EPERM;
 
-		err = mnt_want_write(filp->f_path.mnt);
+		err = mnt_want_write_file(filp);
 		if (err)
 			return err;
 		if (get_user(generation, (int __user *) arg)) {
@@ -192,7 +192,7 @@
 			return -EOPNOTSUPP;
 		}
 
-		err = mnt_want_write(filp->f_path.mnt);
+		err = mnt_want_write_file(filp);
 		if (err)
 			return err;
 
@@ -240,7 +240,7 @@
 			return -EOPNOTSUPP;
 		}
 
-		err = mnt_want_write(filp->f_path.mnt);
+		err = mnt_want_write_file(filp);
 		if (err)
 			goto mext_out;
 
@@ -277,7 +277,7 @@
 			return -EOPNOTSUPP;
 		}
 
-		err = mnt_want_write(filp->f_path.mnt);
+		err = mnt_want_write_file(filp);
 		if (err)
 			return err;
 
@@ -301,7 +301,7 @@
 		if (!inode_owner_or_capable(inode))
 			return -EACCES;
 
-		err = mnt_want_write(filp->f_path.mnt);
+		err = mnt_want_write_file(filp);
 		if (err)
 			return err;
 		/*
@@ -323,7 +323,7 @@
 		if (!inode_owner_or_capable(inode))
 			return -EACCES;
 
-		err = mnt_want_write(filp->f_path.mnt);
+		err = mnt_want_write_file(filp);
 		if (err)
 			return err;
 		err = ext4_alloc_da_blocks(inode);