ubifs: Correctly use tnc_next() in search_dh_cookie()

Commit c877154d fixed an uninitialized variable and optimized
the function to not call tnc_next() in the first iteration of the
loop. While this seemed perfectly legit and wise, it turned out to
be illegal.
If the lookup function does not find an exact match it will rewind
the cursor by 1.
The rewinded cursor will not match the name hash we are looking for
and this results in a spurious -ENOENT.
So we need to move to the next entry in case of an non-exact match,
but not if the match was exact.

While we are here, update the documentation to avoid further confusion.

Cc: Hyunchul Lee <hyc.lee@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: c877154d ("ubifs: Fix uninitialized variable in search_dh_cookie()")
Fixes: 781f675e

 ("ubifs: Fix unlink code wrt. double hash lookups")
Signed-off-by: Richard Weinberger <richard@nod.at>