fs/nfsd/export.c: Adjust error handling code involving auth_domain_put

Once clp is assigned, it never becomes NULL, so we can make a label for it
in the error handling code.  Because the call to path_lookup follows the
call to auth_domain_find, its error handling code should jump to this new
label.

The semantic match that finds this problem is as follows:
(http://www.emn.fr/x-info/coccinelle/)

// <smpl>
@r@
expression x,E;
statement S;
position p1,p2,p3;
@@

(
if ((x = auth_domain_find@p1(...)) == NULL || ...) S
|
x = auth_domain_find@p1(...)
... when != x
if (x == NULL || ...) S
)
<...
if@p3 (...) { ... when != auth_domain_put(x)
                  when != if (x) { ... auth_domain_put(x); ...}
    return@p2 ...;
}
...>
(
return x;
|
return 0;
|
x = E
|
E = x
|
auth_domain_put(x)
)

@exists@
position r.p1,r.p2,r.p3;
expression x;
int ret != 0;
statement S;
@@

* x = auth_domain_find@p1(...)
  <...
* if@p3 (...)
  S
  ...>
* return@p2 \(NULL\|ret\);
// </smpl>

Signed-off-by: Julia Lawall <julia@diku.dk>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>