selftests: net: Use "grep -E" instead of "egrep"

selftests: net: Use "grep -E" instead of "egrep"

The latest version of grep claims the egrep is now obsolete so the build
now contains warnings that look like:
	egrep: warning: egrep is obsolescent; using grep -E
fix this using "grep -E" instead.

  sed -i "s/egrep/grep -E/g" `grep egrep -rwl tools/testing/selftests/net`

Here are the steps to install the latest grep:

  wget http://ftp.gnu.org/gnu/grep/grep-3.8.tar.gz
  tar xf grep-3.8.tar.gz
  cd grep-3.8 && ./configure && make
  sudo make install
  export PATH=/usr/local/bin:$PATH

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Link: https://lore.kernel.org/r/1669864248-829-1-git-send-email-yangtiezhu@loongson.cn
Signed-off-by: Jakub Kicinski <kuba@kernel.org>