Fix seekdir() followed by an immediate telldir().

[jhb]

Summary

If telldir() is called immediately after a call to seekdir(), POSIX
requires the return value of telldir() to equal the value passed to
seekdir(). The current seekdir code with SINGLEUSE enabled breaks
this case as each call to telldir() allocates a new cookie. Instead,
remove the SINGLEUSE code and change telldir() to look for an existing
cookie for the directory's current location rather than always creating
a new cookie.

PR: 121656

Test Plan

Added a simple unit test that does:

l1 = telldir(d);
seekdir(d, l1);
l2 = telldir(d);

and verifies that l1 is equivalent to l2.

Note that we could dispense with the cookies entirely in the unionfs
case and just use dd_loc as the seek value instead.