smbd: Add get_real_filename_at()

smbd: Add get_real_filename_at()

Make get_real_filename() a wrapper.

Right now shadow_copy2 does a fallback to do get_real_filename() on
the twrp=0 tree in case of snapdirseverywhere because snapdirs can be
somewhere deep in the tree, and doing that correctly would be a
full-tree walk. I'd say that snapdirseverywhere is impossible to
implement if you want symlink safety, i.e. careful top-down tree
traversal together with snapdirseverywhere. If you have
snapdirseverywhere you need to pass down the full path very deep down,
which contradicts our fd-based approach we want to take.

Also, I believe that our test does not 100% correctly reflect what
actually is there: My understanding is that if you activate
snapdirseverywhere for example in GPFS, you see all snapshots at every
level (this would need to be verified). Our test does something more
nasty: It creates and tests a specific snapshot only at one place deep
in the directory hierarchy, which makes it impossible to find without
the full path.

This is all a big mess, but for now we need to deal with it. This adds
the twrp=0 fallback to core smbd, but I don't see any other way to do
that properly. And I do want a fd-based getrealfilename....

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