testptp: Add support for testing ptp_clock_info .adjphase callback

Invoke clock_adjtime syscall with tx.modes set with ADJ_OFFSET when testptp
is invoked with a phase adjustment offset value. Support seconds and
nanoseconds for the offset value.

Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Richard Cochran <richardcochran@gmail.com>
Cc: Maciek Machnikowski <maciek@machnikowski.net>
Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>