autofs: use flexible array in ioctl structure

Commit df8fc4e ("kbuild: Enable -fstrict-flex-arrays=3") introduced a warning
for the autofs_dev_ioctl structure:

In function 'check_name',
    inlined from 'validate_dev_ioctl' at fs/autofs/dev-ioctl.c:131:9,
    inlined from '_autofs_dev_ioctl' at fs/autofs/dev-ioctl.c:624:8:
fs/autofs/dev-ioctl.c:33:14: error: 'strchr' reading 1 or more bytes from a region of size 0 [-Werror=stringop-overread]
   33 |         if (!strchr(name, '/'))
      |              ^~~~~~~~~~~~~~~~~
In file included from include/linux/auto_dev-ioctl.h:10,
                 from fs/autofs/autofs_i.h:10,
                 from fs/autofs/dev-ioctl.c:14:
include/uapi/linux/auto_dev-ioctl.h: In function '_autofs_dev_ioctl':
include/uapi/linux/auto_dev-ioctl.h:112:14: note: source object 'path' of size 0
  112 |         char path[0];
      |              ^~~~

This is easily fixed by changing the gnu 0-length array into a c99
flexible array. Since this is a uapi structure, we have to be careful
about possible regressions but this one should be fine as they are
equivalent here. While it would break building with ancient gcc versions
that predate c99, it helps building with --std=c99 and -Wpedantic builds
in user space, as well as non-gnu compilers. This means we probably
also want it fixed in stable kernels.

Cc: [email protected]
Cc: Kees Cook <[email protected]>
Cc: Gustavo A. R. Silva" <[email protected]>
Signed-off-by: Arnd Bergmann <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: arndb

Documentation/filesystems/autofs-mount-control.rst

@@ -196,7 +196,7 @@ information and return operation results::
 		    struct args_ismountpoint	ismountpoint;
 	    };
 
-	    char path[0];
+	    char path[];
     };
 
 The ioctlfd field is a mount point file descriptor of an autofs mount

Documentation/filesystems/autofs.rst

@@ -467,7 +467,7 @@ Each ioctl is passed a pointer to an `autofs_dev_ioctl` structure::
 			struct args_ismountpoint	ismountpoint;
 		};
 
-                char path[0];
+                char path[];
         };
 
 For the **OPEN_MOUNT** and **IS_MOUNTPOINT** commands, the target

include/uapi/linux/auto_dev-ioctl.h

@@ -109,7 +109,7 @@ struct autofs_dev_ioctl {
 		struct args_ismountpoint	ismountpoint;
 	};
 
-	char path[0];
+	char path[];
 };
 
 static inline void init_autofs_dev_ioctl(struct autofs_dev_ioctl *in)