proc: Fix W=1 build kernel-doc warning

proc: Fix W=1 build kernel-doc warning

Building the kernel with W=1 generates the following warning:

  fs/proc/fd.c:81: warning: This comment starts with '/**',
                   but isn't a kernel-doc comment.

Use a normal comment for the helper function proc_fdinfo_permission().

Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Link: https://lore.kernel.org/r/20241018102705.92237-2-thorsten.blum@linux.dev
Signed-off-by: Christian Brauner <brauner@kernel.org>