Squashfs: reject negative file sizes in squashfs_read_inode()

Syskaller reports a "WARNING in ovl_copy_up_file" in overlayfs.

This warning is ultimately caused because the underlying Squashfs file
system returns a file with a negative file size.

This commit checks for a negative file size and returns EINVAL.

[phillip@squashfs.org.uk: only need to check 64 bit quantity]
  Link: https://lkml.kernel.org/r/20250926222305.110103-1-phillip@squashfs.org.uk
Link: https://lkml.kernel.org/r/20250926215935.107233-1-phillip@squashfs.org.uk


Fixes: 6545b246 ("Squashfs: inode operations")
Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Reported-by:  <syzbot+f754e01116421e9754b9@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/68d580e5.a00a0220.303701.0019.GAE@google.com/


Cc: Amir Goldstein <amir73il@gmail.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>