cifs: add comments explaining cifs_new_fileinfo behavior

The comments make it clear the otherwise subtle behavior of cifs_new_fileinfo().

Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
Reviewed-by: Shirish Pargaonkar <shirishp@us.ibm.com>
--
 fs/cifs/dir.c |   18 ++++++++++++++++--
 1 files changed, 16 insertions(+), 2 deletions(-)
Signed-off-by: Steve French <sfrench@us.ibm.com>