net: Introduce recvmmsg socket syscall

Meaning receive multiple messages, reducing the number of syscalls and
net stack entry/exit operations.

Next patches will introduce mechanisms where protocols that want to
optimize this operation will provide an unlocked_recvmsg operation.

This takes into account comments made by:

. Paul Moore: sock_recvmsg is called only for the first datagram,
  sock_recvmsg_nosec is used for the rest.

. Caitlin Bestler: recvmmsg now has a struct timespec timeout, that
  works in the same fashion as the ppoll one.

  If the underlying protocol returns a datagram with MSG_OOB set, this
  will make recvmmsg return right away with as many datagrams (+ the OOB
  one) it has received so far.

. Rémi Denis-Courmont & Steven Whitehouse: If we receive N < vlen
  datagrams and then recvmsg returns an error, recvmmsg will return
  the successfully received datagrams, store the error and return it
  in the next call.

This paves the way for a subsequent optimization, sk_prot->unlocked_recvmsg,
where we will be able to acquire the lock only at batch start and end, not at
every underlying recvmsg call.

Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/mips/kernel/scall32-o32.S

@@ -583,6 +583,7 @@ einval:	li	v0, -ENOSYS
 	sys	sys_rt_tgsigqueueinfo	4
 	sys	sys_perf_event_open	5
 	sys	sys_accept4		4
+	sys     sys_recvmmsg            5
 	.endm
 
 	/* We pre-compute the number of _instruction_ bytes needed to

arch/mips/kernel/scall64-64.S

@@ -420,4 +420,5 @@ sys_call_table:
 	PTR	sys_rt_tgsigqueueinfo
 	PTR	sys_perf_event_open
 	PTR	sys_accept4
+	PTR     sys_recvmmsg
 	.size	sys_call_table,.-sys_call_table

arch/mips/kernel/scall64-n32.S

@@ -418,4 +418,5 @@ EXPORT(sysn32_call_table)
 	PTR	compat_sys_rt_tgsigqueueinfo	/* 5295 */
 	PTR	sys_perf_event_open
 	PTR	sys_accept4
+	PTR     compat_sys_recvmmsg
 	.size	sysn32_call_table,.-sysn32_call_table

arch/mips/kernel/scall64-o32.S

@@ -538,4 +538,5 @@ sys_call_table:
 	PTR	compat_sys_rt_tgsigqueueinfo
 	PTR	sys_perf_event_open
 	PTR	sys_accept4
+	PTR     compat_sys_recvmmsg
 	.size	sys_call_table,.-sys_call_table