proc: more readdir conversion bug-fixes

In the previous commit, Richard Genoud fixed proc_root_readdir(), which
had lost the check for whether all of the non-process /proc entries had
been returned or not.

But that in turn exposed _another_ bug, namely that the original readdir
conversion patch had yet another problem: it had lost the return value
of proc_readdir_de(), so now checking whether it had completed
successfully or not didn't actually work right anyway.

This reinstates the non-zero return for the "end of base entries" that
had also gotten lost in commit f0c3b5093add ("[readdir] convert
procfs").  So now you get all the base entries *and* you get all the
process entries, regardless of getdents buffer size.

(Side note: the Linux "getdents" manual page actually has a nice example
application for testing getdents, which can be easily modified to use
different buffers.  Who knew? Man-pages can be useful)

Reported-by: Emmanuel Benisty <benisty.e@gmail.com>
Reported-by: Marc Dionne <marc.c.dionne@gmail.com>
Cc: Richard Genoud <richard.genoud@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>