smbd: Simplify OpenDir_from_pathref()

smbd: Simplify OpenDir_from_pathref()

Use the /proc/self/fd trick to make get_real_filename_fullscan_at look
a bit nicer and faster in strace. Direct SMB_VFS_OPENAT also is
cheaper in user space, we don't need the full fd_openat and
non_widelink_open magic here.

Also avoid opening ".", which can fail where a full path open would
succeed: If the directory in question does not give "x" perms to the
user, we get a handle on the dir as such but can't cd into it.

I haven't seen real-world cases of this, but one of our tests creates
such a scenario. I have further refactoring in my local tree that make
this patch necessary.

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>