smbd: use fd_openat()

smbd: use fd_openat()

This leaves basically two callers of fd_open():

* open_internal_dirfsp_at()
* smbd_smb2_query_directory_send()

Those will be the only codepaths that use full paths relative to the share root
and so require and use the symlink race protection provided by
non_widelink_open().

Of course, until we actuall start passing real dirfsps to fd_openat(),
fd_openat() internally calls fd_open().

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