net: use compat helper functions in compat_sys_recvmmsg

Use (get|put)_compat_timespec helper functions to simplify the code.

Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>