fs/proc/task_mmu: return -ENOTTY for unknown pagemap ioctls

Commit 52526ca7fd ("fs/proc/task_mmu: implement IOCTL to get and
optionally clear info about PTEs") made do_pagemap_cmd() the ioctl
handler of /proc/<pid>/pagemap.

However, do_pagemap_cmd() returns -EINVAL for every command other than
PAGEMAP_SCAN. Before that commit, pagemap had no ioctl handler, and
the VFS returned -ENOTTY for these commands. As of now, they fail with
EINVAL, so tcgetattr() and isatty() on pagemap report EINVAL instead
of ENOTTY.

The issue can be reproduced with a simple C program:

    #include <fcntl.h>
    #include <stdio.h>
    #include <termios.h>

    int main(void)
    {
            struct termios t;
            int fd = open("/proc/self/pagemap", O_RDONLY);

            if (tcgetattr(fd, &t) < 0)
                    perror("tcgetattr");
            return 0;
    }

Before that commit, the result is:

    tcgetattr: Inappropriate ioctl for device

After that commit, the result is:

    tcgetattr: Invalid argument

This patch will return -ENOIOCTLCMD for unknown commands, as
procfs_procmap_ioctl() in the same file does, so that the VFS turns it
into -ENOTTY.

Link: https://lore.kernel.org/20261023181300.3366487-1-hengyul@cs.unc.edu
Fixes: 52526ca7fd ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Signed-off-by: Hengyu Liang <hengyul@cs.unc.edu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Pedro Falcato <pfalcato@suse.de>
Reviewed-by: Lorenzo Stoakes (ARM) <ljs@kernel.org>
Cc: Liam R. Howlett <liam@infradead.org>
Cc: Vlastimil Babka <vbabka@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Muhammad Usama Anjum <usama.anjum@arm.com>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: Michał Mirosław <mirq-linux@rere.qmqm.pl>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: <stable@vger.kernel.org>

fs/proc/task_mmu.c

@@ -3201,7 +3201,7 @@ static long do_pagemap_cmd(struct file *file, unsigned int cmd,
 		return do_pagemap_scan(mm, arg);
 
 	default:
-		return -EINVAL;
+		return -ENOIOCTLCMD;
 	}
 }