docs: networking: convert timestamping.txt to ReST

- add SPDX header;
- add a document title;
- adjust titles and chapters, adding proper markups;
- mark code blocks and literals as such;
- adjust identation, whitespaces and blank lines where needed;
- add to networking/index.rst.
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/index.rst

@@ -109,6 +109,7 @@ Contents:
    tc-actions-env-rules
    tcp-thin
    team
+   timestamping
 
 .. only::  subproject and html
 

Documentation/networking/packet_mmap.rst

@@ -1030,7 +1030,7 @@ the packet meta information for mmap(2)ed RX_RING and TX_RINGs.  If your
 NIC is capable of timestamping packets in hardware, you can request those
 hardware timestamps to be used. Note: you may need to enable the generation
 of hardware timestamps with SIOCSHWTSTAMP (see related information from
-Documentation/networking/timestamping.txt).
+Documentation/networking/timestamping.rst).
 
 PACKET_TIMESTAMP accepts the same integer bit field as SO_TIMESTAMPING::
 
@@ -1069,7 +1069,7 @@ TX_RING part only TP_STATUS_AVAILABLE is set, then the tp_sec and tp_{n,u}sec
 members do not contain a valid value. For TX_RINGs, by default no timestamp
 is generated!
 
-See include/linux/net_tstamp.h and Documentation/networking/timestamping.txt
+See include/linux/net_tstamp.h and Documentation/networking/timestamping.rst
 for more information on hardware timestamps.
 
 Miscellaneous bits

include/uapi/linux/errqueue.h

@@ -36,7 +36,7 @@ struct sock_extended_err {
  *
  *	The timestamping interfaces SO_TIMESTAMPING, MSG_TSTAMP_*
  *	communicate network timestamps by passing this struct in a cmsg with
- *	recvmsg(). See Documentation/networking/timestamping.txt for details.
+ *	recvmsg(). See Documentation/networking/timestamping.rst for details.
  *	User space sees a timespec definition that matches either
  *	__kernel_timespec or __kernel_old_timespec, in the kernel we
  *	require two structure definitions to provide both.