s3/smbd: let non_widelink_open() chdir() to directories directly

s3/smbd: let non_widelink_open() chdir() to directories directly

If the caller passes O_DIRECTORY we just try to chdir() to smb_fname
directly, not to the parent directory.

The security check in check_reduced_name() will continue to work, but
this fixes the case of an open() for a previous version of a
subdirectory that contains snapshopt.

Eg:

[share]
    path = /shares/test
    vfs objects = shadow_copy2
    shadow:snapdir = .snapshots
    shadow:snapdirseverywhere = yes

Directory tree with fake snapshots:

$ tree -a /shares/test/
/shares/test/
├── dir
│   ├── file
│   └── .snapshots
│       └── @GMT-2017.07.04-04.30.12
│           └── file
├── dir2
│   └── file
├── file
├── .snapshots
│   └── @GMT-2001.01.01-00.00.00
│       ├── dir2
│       │   └── file
│       └── file
└── testfsctl.dat

./bin/smbclient -U slow%x //localhost/share -c 'ls @GMT-2017.07.04-04.30.12/dir/*'
NT_STATUS_OBJECT_NAME_NOT_FOUND listing \@GMT-2017.07.04-04.30.12\dir\*

Bug: https://bugzilla.samba.org/show_bug.cgi?id=12885

Signed-off-by: Ralph Boehme <slow@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>