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/nfsd/nfs4recover.c b/fs/nfsd/nfs4recover.c
index a9aa2f1..80a0be9 100644
--- a/fs/nfsd/nfs4recover.c
+++ b/fs/nfsd/nfs4recover.c
@@ -151,7 +151,7 @@
 	if (status)
 		goto out_put;
 	status = vfs_mkdir(dir->d_inode, dentry, S_IRWXU);
-	mnt_drop_write(rec_file->f_path.mnt);
+	mnt_drop_write_file(rec_file);
 out_put:
 	dput(dentry);
 out_unlock:
@@ -281,7 +281,7 @@
 	nfs4_reset_creds(original_cred);
 	if (status == 0)
 		vfs_fsync(rec_file, 0);
-	mnt_drop_write(rec_file->f_path.mnt);
+	mnt_drop_write_file(rec_file);
 out:
 	if (status)
 		printk("NFSD: Failed to remove expired client state directory"
@@ -317,7 +317,7 @@
 	status = nfsd4_list_rec_dir(purge_old);
 	if (status == 0)
 		vfs_fsync(rec_file, 0);
-	mnt_drop_write(rec_file->f_path.mnt);
+	mnt_drop_write_file(rec_file);
 out:
 	if (status)
 		printk("nfsd4: failed to purge old clients from recovery"