nfsd: fix possible oops on re-insertion of rpcsec_gss modules

nfsd: fix possible oops on re-insertion of rpcsec_gss modules

The handling of the re-registration case is wrong here; the "test" that was
returned from auth_domain_lookup will not be used again, so that reference
should be put.  And auth_domain_lookup never did anything with "new" in
this case, so we should just clean it up ourself.

Thanks to Akinobu Mita for bug report, analysis, and testing.

Cc: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>
Cc: Neil Brown <neilb@suse.de>
Cc: Trond Myklebust <trond.myklebust@fys.uio.no>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>