fs: add ksys_rmdir() wrapper; remove in-kernel calls to sys_rmdir()

Using this wrapper allows us to avoid the in-kernel calls to the
sys_rmdir() syscall. The ksys_ prefix denotes that this function is meant
as a drop-in replacement for the syscall. In particular, it uses the same
calling convention as sys_rmdir().

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: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Dominik Brodowski <linux@dominikbrodowski.net>

fs/internal.h

@@ -55,6 +55,7 @@ extern void __init chrdev_init(void);
 extern int user_path_mountpoint_at(int, const char __user *, unsigned int, struct path *);
 extern int vfs_path_lookup(struct dentry *, struct vfsmount *,
 			   const char *, unsigned int, struct path *);
+long do_rmdir(int dfd, const char __user *pathname);
 long do_unlinkat(int dfd, struct filename *name);
 
 /*

fs/namei.c

@@ -3872,7 +3872,7 @@ int vfs_rmdir(struct inode *dir, struct dentry *dentry)
 }
 EXPORT_SYMBOL(vfs_rmdir);
 
-static long do_rmdir(int dfd, const char __user *pathname)
+long do_rmdir(int dfd, const char __user *pathname)
 {
 	int error = 0;
 	struct filename *name;

include/linux/syscalls.h

@@ -965,4 +965,11 @@ static inline long ksys_unlink(const char __user *pathname)
 	return do_unlinkat(AT_FDCWD, getname(pathname));
 }
 
+extern long do_rmdir(int dfd, const char __user *pathname);
+
+static inline long ksys_rmdir(const char __user *pathname)
+{
+	return do_rmdir(AT_FDCWD, pathname);
+}
+
 #endif

init/initramfs.c

@@ -317,7 +317,7 @@ static void __init clean_path(char *path, umode_t fmode)
 
 	if (!vfs_lstat(path, &st) && (st.mode ^ fmode) & S_IFMT) {
 		if (S_ISDIR(st.mode))
-			sys_rmdir(path);
+			ksys_rmdir(path);
 		else
 			ksys_unlink(path);
 	}
@@ -589,7 +589,7 @@ static void __init clean_rootfs(void)
 			WARN_ON_ONCE(ret);
 			if (!ret) {
 				if (S_ISDIR(st.mode))
-					sys_rmdir(dirp->d_name);
+					ksys_rmdir(dirp->d_name);
 				else
 					ksys_unlink(dirp->d_name);
 			}