vfs: make O_PATH file descriptors usable for 'fchdir()'

We already use them for openat() and friends, but fchdir() also wants to
be able to use O_PATH file descriptors.  This should make it comparable
to the O_SEARCH of Solaris.  In particular, O_PATH allows you to access
(not-quite-open) a directory you don't have read persmission to, only
execute permission.

Noticed during development of multithread support for ksh93.

Reported-by: ольга крыжановская <olga.kryzhanovska@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: stable@kernel.org    # O_PATH introduced in 3.0+
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/open.c

@@ -397,10 +397,10 @@ SYSCALL_DEFINE1(fchdir, unsigned int, fd)
 {
 	struct file *file;
 	struct inode *inode;
-	int error;
+	int error, fput_needed;
 
 	error = -EBADF;
-	file = fget(fd);
+	file = fget_raw_light(fd, &fput_needed);
 	if (!file)
 		goto out;
 
@@ -414,7 +414,7 @@ SYSCALL_DEFINE1(fchdir, unsigned int, fd)
 	if (!error)
 		set_fs_pwd(current->fs, &file->f_path);
 out_putf:
-	fput(file);
+	fput_light(file, fput_needed);
 out:
 	return error;
 }