svcrpc: fix bad argument in unix_domain_find

"After merging the nfsd tree, today's linux-next build (powerpc
ppc64_defconfig) produced this warning:

net/sunrpc/svcauth_unix.c: In function 'unix_domain_find':
net/sunrpc/svcauth_unix.c:58: warning: passing argument 1 of
+'svcauth_unix_domain_release' from incompatible pointer type
net/sunrpc/svcauth_unix.c:41: note: expected 'struct auth_domain *' but
argument
+is of type 'struct unix_domain *'

Introduced by commit 8b3e07ac

 ("svcrpc: fix rare race on unix_domain
creation")."

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>