selftests: mptcp: info: hide 'grep: write error' warnings

mptcp_lib_get_info_value() will only print the first entry that match
the filter because of the ';q' at the end. As a consequence, the 'sed'
command could finish before the previous 'grep' one and print a 'write
error' warning because it is trying to write data to the closed pipe.

Such warnings are not interesting, they can be hidden by muting stderr
here for grep.

While at it, clearly indicate that mptcp_lib_get_info_value() will only
print the first matched entry to avoid confusions later on.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20250502-net-next-mptcp-sft-inc-cover-v1-1-68eec95898fb@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>