cifs: make cifs_ioctl handle NULL filp->private_data correctly

Commit 13cfb733 made cifs_ioctl use the tlink attached to the
cifsFileInfo for a filp. This ignores the case of an open directory
however, which in CIFS can have a NULL private_data until a readdir
is done on it.

This patch re-adds the NULL pointer checks that were removed in commit
50ae28f0

 and moves the setting of tcon and "caps" variables lower.

Long term, a better fix would be to establish a f_op->open routine for
directories that populates that field at open time, but that requires
some other changes to how readdir calls are handled.

Reported-by: Kjell Rune Skaaraas <kjella79@yahoo.no>
Reviewed-and-Tested-by: Suresh Jayaraman <sjayaraman@suse.de>
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>