fat (exportfs): fix some kernel-doc warnings

fat (exportfs): fix some kernel-doc warnings

Fix the following W=1 kernel build warning(s):

  fs/fat/nfs.c:21: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst
  fs/fat/nfs.c:139: warning: This comment starts with '/**', but isn't a kernel-doc comment. Refer Documentation/doc-guide/kernel-doc.rst

Link: https://lkml.kernel.org/r/20221111075648.4005-1-liubo03@inspur.com
Signed-off-by: Bo Liu <liubo03@inspur.com>
Acked-by: OGAWA Hirofumi <hirofumi@mail.parknet.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>