rtpsink: Return proper pad from _request_new_pad

Bizarrely, it returned a pad from the child rtpbin. I noticed because
our application leaked the implicitly created ghost pad. Make an
explicit ghost pad so this works properly.

Part-of: <gstreamer/gst-plugins-bad!2250>
29 jobs for !2250 with backport-mr-2227-into-1.18-rtp-pad-leak in 26 minutes and 54 seconds (queued for 2 seconds)
detached
Status Name Job ID Coverage
  Build Docker
passed alpine amd64 manifest builder docker #9780830

00:00:49

passed android docker #9780829

00:00:50

passed cerbero fedora amd64 docker #9780828

00:00:29

passed fedora amd64 docker #9780827

00:00:41

passed gst-indent amd64 docker #9780831

00:00:38

 
  Preparation
passed gst indent #9780833

00:00:13

passed manifest #9780832

00:00:22

 
  Build
passed build cerbero cross-android universal #9780842
gstreamer

00:06:58

passed build cerbero cross-ios universal #9780846
gst-ios-14.3

00:21:06

passed build cerbero cross win32 #9780843
gstreamer

00:04:16

passed build cerbero cross win64 #9780844
gstreamer

00:05:06

passed build cerbero fedora x86_64 #9780841
gstreamer

00:04:19

passed build cerbero macos x86_64 #9780845
gst-macos-11.1

00:11:52

passed build clang fedora x86_64 #9780837

00:04:53

manual build msys2 #9780840
docker windows 1809 allowed to fail manual
passed build nodebug fedora x86_64 #9780834

00:04:55

passed build static fedora x86_64 #9780835

00:04:55

passed build static nodebug fedora x86_64 #9780836

00:04:54

passed build vs2017 amd64 #9780838
docker windows 1809

00:11:03

passed build vs2017 x86 #9780839
docker windows 1809

00:10:42

 
  Test
passed check fedora #9780847
gstreamer

00:06:57

passed integration testsuites fedora 1/4 #9780848
gstreamer

00:06:45

passed integration testsuites fedora 2/4 #9780849
gstreamer

00:07:33

passed integration testsuites fedora 3/4 #9780850
gstreamer

00:06:56

passed integration testsuites fedora 4/4 #9780851
gstreamer

00:06:51

passed valgrind bad #9780852
gstreamer

00:06:19

 
  Integrate
passed cross-android universal examples #9780853

00:07:43

passed cross-ios universal examples #9780855
gst-ios-14.3

00:04:25

passed documentation #9780854

00:11:25