fs: add do_renameat2() helper; remove internal call to sys_renameat2()

Using this helper removes in-kernel calls to the sys_renameat2() syscall.

This patch is part of a series which removes in-kernel calls to syscalls.
On this basis, the syscall entry path can be streamlined. For details, see
http://lkml.kernel.org/r/20180325162527.GA17492@light.dominikbrodowski.net

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

fs/namei.c

@@ -4478,8 +4478,8 @@ int vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
 }
 EXPORT_SYMBOL(vfs_rename);
 
-SYSCALL_DEFINE5(renameat2, int, olddfd, const char __user *, oldname,
-		int, newdfd, const char __user *, newname, unsigned int, flags)
+static int do_renameat2(int olddfd, const char __user *oldname, int newdfd,
+			const char __user *newname, unsigned int flags)
 {
 	struct dentry *old_dentry, *new_dentry;
 	struct dentry *trap;
@@ -4621,15 +4621,21 @@ SYSCALL_DEFINE5(renameat2, int, olddfd, const char __user *, oldname,
 	return error;
 }
 
+SYSCALL_DEFINE5(renameat2, int, olddfd, const char __user *, oldname,
+		int, newdfd, const char __user *, newname, unsigned int, flags)
+{
+	return do_renameat2(olddfd, oldname, newdfd, newname, flags);
+}
+
 SYSCALL_DEFINE4(renameat, int, olddfd, const char __user *, oldname,
 		int, newdfd, const char __user *, newname)
 {
-	return sys_renameat2(olddfd, oldname, newdfd, newname, 0);
+	return do_renameat2(olddfd, oldname, newdfd, newname, 0);
 }
 
 SYSCALL_DEFINE2(rename, const char __user *, oldname, const char __user *, newname)
 {
-	return sys_renameat2(AT_FDCWD, oldname, AT_FDCWD, newname, 0);
+	return do_renameat2(AT_FDCWD, oldname, AT_FDCWD, newname, 0);
 }
 
 int vfs_whiteout(struct inode *dir, struct dentry *dentry)