selftests/filesystems: Assume that TIOCGPTPEER is defined

selftests/filesystems: Assume that TIOCGPTPEER is defined

The devpts_pts selftest has an ifdef in case an architecture does not
define TIOCGPTPEER, but the handling for this is broken since we need
errno to be set to EINVAL in order to skip the test as we should. Given
that this ioctl() has been defined since v4.15 we may as well just assume
it's there rather than write handling code which will probably never get
used.

Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://patch.msgid.link/20260227-selftests-filesystems-devpts-tiocgptpeer-v3-1-07db4d85d5aa@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>