lockd: fix reference count leaks in async locking case

In a number of places where we wish only to translate nlm_drop_reply to
rpc_drop_reply errors we instead return early with rpc_drop_reply,
skipping some important end-of-function cleanup.

This results in reference count leaks when lockd is doing posix locking
on GFS2.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>