svcrpc: fix handling of garbage args

To return garbage_args, the accept_stat must be 0, and we must have a
verifier.  So we shouldn't be resetting the write pointer as we reject
the call.

Also, we must add the two placeholder words here regardless of success
of the unwrap, to ensure the output buffer is left in a consistent state
for svcauth_gss_release().

This fixes a BUG() in svcauth_gss.c:svcauth_gss_release().

Thanks to Aime Le Rouzic for bug report, debugging help, and testing.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Tested-by: Aime Le Rouzic <aime.le-rouzic@bull.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>