staging: lustre: ptlrpc: avoid warning on missing return

The newly added function triggers a harmless warning:

drivers/staging/lustre/lustre/ptlrpc/pack_generic.c: In function 'lustre_shrink_msg':
drivers/staging/lustre/lustre/ptlrpc/pack_generic.c:472:1: error: control reaches end of non-void function [-Werror=return-type]

This probably happens because LASSERTF() contains an 'unlikely()' that
sometimes prevents gcc from analysing the control flow correctly.
Adding a return statement here seems harmless and lets us keep that
unlikely().

Fixes: 96049bd1

 ("staging: lustre: ptlrpc: embed highest XID in each request")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>